diff --git a/oc_ocdm/__init__.py b/oc_ocdm/__init__.py
index 6078e86..287d67d 100644
--- a/oc_ocdm/__init__.py
+++ b/oc_ocdm/__init__.py
@@ -13,7 +13,7 @@
 # DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
 # ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
 # SOFTWARE.
-__version__ = '7.0.1'
+__version__ = '9.1.1'
 
 from oc_ocdm.storer import Storer
 from oc_ocdm.reader import Reader
diff --git a/oc_ocdm/graph/graph_entity.py b/oc_ocdm/graph/graph_entity.py
index caf9259..a201f68 100644
--- a/oc_ocdm/graph/graph_entity.py
+++ b/oc_ocdm/graph/graph_entity.py
@@ -17,9 +17,8 @@
 
 from typing import TYPE_CHECKING
 
-from rdflib import Graph, Namespace, URIRef
-
 from oc_ocdm.abstract_entity import AbstractEntity
+from rdflib import Graph, Namespace, URIRef
 
 if TYPE_CHECKING:
     from typing import ClassVar, Dict, List, Optional, Tuple
diff --git a/oc_ocdm/graph/graph_set.py b/oc_ocdm/graph/graph_set.py
index 36ce793..4236db4 100755
--- a/oc_ocdm/graph/graph_set.py
+++ b/oc_ocdm/graph/graph_set.py
@@ -17,18 +17,15 @@
 
 from typing import TYPE_CHECKING
 
-from SPARQLWrapper import RDFXML, SPARQLWrapper
-
 from oc_ocdm.abstract_set import AbstractSet
 from oc_ocdm.reader import Reader
-from oc_ocdm.support.support import get_count, get_short_name, get_prefix
+from oc_ocdm.support.support import get_count, get_prefix, get_short_name
+from SPARQLWrapper import RDFXML, SPARQLWrapper
 
 if TYPE_CHECKING:
     from typing import Dict, ClassVar, Tuple, Optional, List, Set
     from rdflib import ConjunctiveGraph
 
-from rdflib import Graph, Namespace, URIRef
-
 from oc_ocdm.counter_handler.counter_handler import CounterHandler
 from oc_ocdm.counter_handler.filesystem_counter_handler import \
     FilesystemCounterHandler
@@ -53,6 +50,7 @@
     ResponsibleAgent
 from oc_ocdm.graph.entities.identifier import Identifier
 from oc_ocdm.graph.graph_entity import GraphEntity
+from rdflib import Graph, Namespace, URIRef
 
 
 class GraphSet(AbstractSet):
diff --git a/oc_ocdm/storer.py b/oc_ocdm/storer.py
index 3c10513..30a953c 100644
--- a/oc_ocdm/storer.py
+++ b/oc_ocdm/storer.py
@@ -22,9 +22,7 @@
 from typing import TYPE_CHECKING
 from zipfile import ZIP_DEFLATED, ZipFile
 
-from rdflib import ConjunctiveGraph, URIRef
-from SPARQLWrapper import SPARQLWrapper
-
+from filelock import FileLock
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.metadata.metadata_entity import MetadataEntity
 from oc_ocdm.prov.prov_entity import ProvEntity
@@ -32,7 +30,8 @@
 from oc_ocdm.support.query_utils import get_update_query
 from oc_ocdm.support.reporter import Reporter
 from oc_ocdm.support.support import find_paths
-from filelock import FileLock
+from rdflib import ConjunctiveGraph, URIRef
+from SPARQLWrapper import SPARQLWrapper
 
 if TYPE_CHECKING:
     from typing import Any, Dict, List, Optional, Set, Tuple
diff --git a/oc_ocdm/support/support.py b/oc_ocdm/support/support.py
index a89468e..803dae3 100644
--- a/oc_ocdm/support/support.py
+++ b/oc_ocdm/support/support.py
@@ -156,6 +156,7 @@ def encode_url(u: str) -> str:
 
 def create_literal(g: Graph, res: URIRef, p: URIRef, s: str, dt: URIRef = None, nor: bool = True) -> None:
     if not is_string_empty(s):
+        dt = dt if dt is not None else XSD.string
         g.add((res, p, Literal(s, datatype=dt, normalize=nor)))
 
 
@@ -371,7 +372,7 @@ def build_graph_from_results(results: List[Dict]) -> Graph:
             o = URIRef(triple['o']['value'])
         else:
             datatype = triple['o'].get('datatype', None)
-            datatype = URIRef(datatype) if datatype is not None else None
+            datatype = URIRef(datatype) if datatype is not None else XSD.string
             o = Literal(triple['o']['value'], datatype=datatype)
         graph.add((s, p, o))
     return graph
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_reference.py b/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_reference.py
index 365d268..58266ba 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_reference.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_reference.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import Literal
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import XSD, Literal
 
 
 class TestBibliographicReference(unittest.TestCase):
@@ -38,7 +37,7 @@ def test_has_content(self):
         result = self.be.has_content(content)
         self.assertIsNone(result)
 
-        triple = self.be.res, GraphEntity.iri_has_content, Literal(content)
+        triple = self.be.res, GraphEntity.iri_has_content, Literal(content, datatype=XSD.string)
         self.assertIn(triple, self.be.g)
 
     def test_has_annotation(self):
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_resource.py b/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_resource.py
index af38441..dea9a3d 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_resource.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_bibliographic_resource.py
@@ -41,7 +41,7 @@ def test_has_title(self):
         result = self.br1.has_title(title)
         self.assertIsNone(result)
 
-        triple = self.br1.res, GraphEntity.iri_title, Literal(title)
+        triple = self.br1.res, GraphEntity.iri_title, Literal(title, datatype=XSD.string)
         self.assertIn(triple, self.br1.g)
 
     def test_has_subtitle(self):
@@ -49,7 +49,7 @@ def test_has_subtitle(self):
         result = self.br1.has_subtitle(subtitle)
         self.assertIsNone(result)
 
-        triple = self.br1.res, GraphEntity.iri_has_subtitle, Literal(subtitle)
+        triple = self.br1.res, GraphEntity.iri_has_subtitle, Literal(subtitle, datatype=XSD.string)
         self.assertIn(triple, self.br1.g)
 
     def test_is_part_of(self):
@@ -107,7 +107,7 @@ def test_create_number(self):
         result = self.br1.has_number(number)
         self.assertIsNone(result)
 
-        triple = self.br1.res, GraphEntity.iri_has_sequence_identifier, Literal(number)
+        triple = self.br1.res, GraphEntity.iri_has_sequence_identifier, Literal(number, datatype=XSD.string)
         self.assertIn(triple, self.br1.g)
 
     def test_has_edition(self):
@@ -115,7 +115,7 @@ def test_has_edition(self):
         result = self.br1.has_edition(edition)
         self.assertIsNone(result)
 
-        triple = self.br1.res, GraphEntity.iri_has_edition, Literal(edition)
+        triple = self.br1.res, GraphEntity.iri_has_edition, Literal(edition, datatype=XSD.string)
         self.assertIn(triple, self.br1.g)
 
     def test_contains_in_reference_list(self):
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_discourse_element.py b/oc_ocdm/test/graph/entities/bibliographic/test_discourse_element.py
index 0ef61ca..02bfa36 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_discourse_element.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_discourse_element.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import Literal, RDF
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import RDF, XSD, Literal
 
 
 class TestDiscourseElement(unittest.TestCase):
@@ -39,7 +38,7 @@ def test_has_title(self):
         result = self.de1.has_title(title)
         self.assertIsNone(result)
 
-        triple = self.de1.res, GraphEntity.iri_title, Literal(title)
+        triple = self.de1.res, GraphEntity.iri_title, Literal(title, datatype=XSD.string)
         self.assertIn(triple, self.de1.g)
 
     def test_contains_discourse_element(self):
@@ -75,7 +74,7 @@ def test_has_content(self):
         result = self.de1.has_content(content)
         self.assertIsNone(result)
 
-        triple = self.de1.res, GraphEntity.iri_has_content, Literal(content)
+        triple = self.de1.res, GraphEntity.iri_has_content, Literal(content, datatype=XSD.string)
         self.assertIn(triple, self.de1.g)
 
     def test_create_section(self):
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_pointer_list.py b/oc_ocdm/test/graph/entities/bibliographic/test_pointer_list.py
index 79f9c82..ac01435 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_pointer_list.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_pointer_list.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import Literal
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import XSD, Literal
 
 
 class TestPointerList(unittest.TestCase):
@@ -37,7 +36,7 @@ def test_has_content(self):
         result = self.pl.has_content(content)
         self.assertIsNone(result)
 
-        triple = self.pl.res, GraphEntity.iri_has_content, Literal(content)
+        triple = self.pl.res, GraphEntity.iri_has_content, Literal(content, datatype=XSD.string)
         self.assertIn(triple, self.pl.g)
 
     def test_contains_element(self):
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_reference_pointer.py b/oc_ocdm/test/graph/entities/bibliographic/test_reference_pointer.py
index 01c8c71..ccd981d 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_reference_pointer.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_reference_pointer.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import Literal
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import XSD, Literal
 
 
 class TestReferencePointer(unittest.TestCase):
@@ -39,7 +38,7 @@ def test_has_content(self):
         result = self.rp1.has_content(content)
         self.assertIsNone(result)
 
-        triple = self.rp1.res, GraphEntity.iri_has_content, Literal(content)
+        triple = self.rp1.res, GraphEntity.iri_has_content, Literal(content, datatype=XSD.string)
         self.assertIn(triple, self.rp1.g)
 
     def test_has_next_rp(self):
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_resource_embodiment.py b/oc_ocdm/test/graph/entities/bibliographic/test_resource_embodiment.py
index 6eb55a2..a8d1a69 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_resource_embodiment.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_resource_embodiment.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import URIRef, Literal, RDF
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import RDF, XSD, Literal, URIRef
 
 
 class TestResourceEmbodiment(unittest.TestCase):
@@ -44,7 +43,7 @@ def test_has_starting_page(self):
         result = self.re.has_starting_page(starting_page)
         self.assertIsNone(result)
 
-        triple = self.re.res, GraphEntity.iri_starting_page, Literal(starting_page)
+        triple = self.re.res, GraphEntity.iri_starting_page, Literal(starting_page, datatype=XSD.string)
         self.assertIn(triple, self.re.g)
 
     def test_has_ending_page(self):
@@ -52,7 +51,7 @@ def test_has_ending_page(self):
         result = self.re.has_ending_page(ending_page)
         self.assertIsNone(result)
 
-        triple = self.re.res, GraphEntity.iri_ending_page, Literal(ending_page)
+        triple = self.re.res, GraphEntity.iri_ending_page, Literal(ending_page, datatype=XSD.string)
         self.assertIn(triple, self.re.g)
 
     def test_has_url(self):
diff --git a/oc_ocdm/test/graph/entities/bibliographic/test_responsible_agent.py b/oc_ocdm/test/graph/entities/bibliographic/test_responsible_agent.py
index 2535937..285d4fa 100644
--- a/oc_ocdm/test/graph/entities/bibliographic/test_responsible_agent.py
+++ b/oc_ocdm/test/graph/entities/bibliographic/test_responsible_agent.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import Literal, URIRef
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import XSD, Literal, URIRef
 
 
 class TestResponsibleAgent(unittest.TestCase):
@@ -37,7 +36,7 @@ def test_has_name(self):
         result = self.ra.has_name(name)
         self.assertIsNone(result)
 
-        triple = self.ra.res, GraphEntity.iri_name, Literal(name)
+        triple = self.ra.res, GraphEntity.iri_name, Literal(name, datatype=XSD.string)
         self.assertIn(triple, self.ra.g)
 
     def test_has_given_name(self):
@@ -45,7 +44,7 @@ def test_has_given_name(self):
         result = self.ra.has_given_name(given_name)
         self.assertIsNone(result)
 
-        triple = self.ra.res, GraphEntity.iri_given_name, Literal(given_name)
+        triple = self.ra.res, GraphEntity.iri_given_name, Literal(given_name, datatype=XSD.string)
         self.assertIn(triple, self.ra.g)
 
     def test_has_family_name(self):
@@ -53,7 +52,7 @@ def test_has_family_name(self):
         result = self.ra.has_family_name(family_name)
         self.assertIsNone(result)
 
-        triple = self.ra.res, GraphEntity.iri_family_name, Literal(family_name)
+        triple = self.ra.res, GraphEntity.iri_family_name, Literal(family_name, datatype=XSD.string)
         self.assertIn(triple, self.ra.g)
 
     def test_has_related_agent(self):
diff --git a/oc_ocdm/test/graph/entities/test_identifier.py b/oc_ocdm/test/graph/entities/test_identifier.py
index cb15184..4cd765a 100644
--- a/oc_ocdm/test/graph/entities/test_identifier.py
+++ b/oc_ocdm/test/graph/entities/test_identifier.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import Literal
-
 from oc_ocdm.graph.graph_entity import GraphEntity
 from oc_ocdm.graph.graph_set import GraphSet
+from rdflib import XSD, Literal
 
 
 class TestIdentifier(unittest.TestCase):
@@ -36,7 +35,7 @@ def test_create_orcid(self):
         result = self.identifier.create_orcid(orcid)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(orcid)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(orcid, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_orcid
@@ -47,7 +46,7 @@ def test_create_doi(self):
         result = self.identifier.create_doi(doi)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(doi)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(doi, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_doi
@@ -58,7 +57,7 @@ def test_create_pmid(self):
         result = self.identifier.create_pmid(pmid)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(pmid)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(pmid, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_pmid
@@ -69,7 +68,7 @@ def test_create_pmcid(self):
         result = self.identifier.create_pmcid(pmcid)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(pmcid)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(pmcid, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_pmcid
@@ -80,7 +79,7 @@ def test_create_issn(self):
         result = self.identifier.create_issn(issn)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(issn)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(issn, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_issn
@@ -91,7 +90,7 @@ def test_create_isbn(self):
         result = self.identifier.create_isbn(isbn)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(isbn)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(isbn, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_isbn
@@ -102,7 +101,7 @@ def test_create_url(self):
         result = self.identifier.create_url(url)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(url)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(url, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_url
@@ -113,7 +112,7 @@ def test_create_xpath(self):
         result = self.identifier.create_xpath(xpath)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(xpath)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(xpath, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_xpath
@@ -124,7 +123,7 @@ def test_create_intrepid(self):
         result = self.identifier.create_intrepid(intrepid)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(intrepid)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(intrepid, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_intrepid
@@ -135,7 +134,7 @@ def test_create_xmlid(self):
         result = self.identifier.create_xmlid(xmlid)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(xmlid)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(xmlid, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_xmlid
@@ -146,7 +145,7 @@ def test_create_wikidata(self):
         result = self.identifier.create_wikidata(wikidata)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(wikidata)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(wikidata, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_wikidata
@@ -157,7 +156,7 @@ def test_create_wikipedia(self):
         result = self.identifier.create_wikipedia(wikipedia)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(wikipedia)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(wikipedia, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_wikipedia
@@ -168,7 +167,7 @@ def test_create_crossref(self):
         result = self.identifier.create_crossref(crossref)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(crossref)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(crossref, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_crossref
@@ -179,7 +178,7 @@ def test_create_datacite(self):
         result = self.identifier.create_datacite(datacite)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(datacite)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(datacite, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_datacite
@@ -190,7 +189,7 @@ def test_create_viaf(self):
         result = self.identifier.create_viaf(viaf)
         self.assertIsNone(result)
 
-        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(viaf)
+        triple = self.identifier.res, GraphEntity.iri_has_literal_value, Literal(viaf, datatype=XSD.string)
         self.assertIn(triple, self.identifier.g)
 
         triple = self.identifier.res, GraphEntity.iri_uses_identifier_scheme, GraphEntity.iri_viaf
diff --git a/oc_ocdm/test/metadata/entities/test_dataset.py b/oc_ocdm/test/metadata/entities/test_dataset.py
index cf15b75..ca943ab 100644
--- a/oc_ocdm/test/metadata/entities/test_dataset.py
+++ b/oc_ocdm/test/metadata/entities/test_dataset.py
@@ -15,10 +15,9 @@
 # SOFTWARE.
 import unittest
 
-from rdflib import URIRef, Literal, XSD
-
-from oc_ocdm.metadata.metadata_set import MetadataSet
 from oc_ocdm.metadata.metadata_entity import MetadataEntity
+from oc_ocdm.metadata.metadata_set import MetadataSet
+from rdflib import XSD, Literal, URIRef
 
 
 class TestDataset(unittest.TestCase):
@@ -38,7 +37,7 @@ def test_has_title(self):
         result = self.dataset.has_title(title)
         self.assertIsNone(result)
 
-        triple = self.dataset.res, MetadataEntity.iri_title, Literal(title)
+        triple = self.dataset.res, MetadataEntity.iri_title, Literal(title, datatype=XSD.string)
         self.assertIn(triple, self.dataset.g)
 
     def test_has_description(self):
@@ -46,7 +45,7 @@ def test_has_description(self):
         result = self.dataset.has_description(description)
         self.assertIsNone(result)
 
-        triple = self.dataset.res, MetadataEntity.iri_description, Literal(description)
+        triple = self.dataset.res, MetadataEntity.iri_description, Literal(description, datatype=XSD.string)
         self.assertIn(triple, self.dataset.g)
 
     def test_has_publication_date(self):
@@ -72,7 +71,7 @@ def test_has_keyword(self):
         result = self.dataset.has_keyword(keyword)
         self.assertIsNone(result)
 
-        triple = self.dataset.res, MetadataEntity.iri_keyword, Literal(keyword)
+        triple = self.dataset.res, MetadataEntity.iri_keyword, Literal(keyword, datatype=XSD.string)
         self.assertIn(triple, self.dataset.g)
 
     def test_has_subject(self):
diff --git a/oc_ocdm/test/metadata/entities/test_distribution.py b/oc_ocdm/test/metadata/entities/test_distribution.py
index d305261..d59a0cb 100644
--- a/oc_ocdm/test/metadata/entities/test_distribution.py
+++ b/oc_ocdm/test/metadata/entities/test_distribution.py
@@ -36,7 +36,7 @@ def test_has_title(self):
         result = self.di.has_title(title)
         self.assertIsNone(result)
 
-        triple = self.di.res, MetadataEntity.iri_title, Literal(title)
+        triple = self.di.res, MetadataEntity.iri_title, Literal(title, datatype=XSD.string)
         self.assertIn(triple, self.di.g)
 
     def test_has_description(self):
@@ -44,7 +44,7 @@ def test_has_description(self):
         result = self.di.has_description(description)
         self.assertIsNone(result)
 
-        triple = self.di.res, MetadataEntity.iri_description, Literal(description)
+        triple = self.di.res, MetadataEntity.iri_description, Literal(description, datatype=XSD.string)
         self.assertIn(triple, self.di.g)
 
     def test_has_publication_date(self):
diff --git a/oc_ocdm/test/prov/entities/test_snapshot_entity.py b/oc_ocdm/test/prov/entities/test_snapshot_entity.py
index 9934993..b8551c9 100644
--- a/oc_ocdm/test/prov/entities/test_snapshot_entity.py
+++ b/oc_ocdm/test/prov/entities/test_snapshot_entity.py
@@ -83,7 +83,7 @@ def test_has_update_action(self):
         result = self.se.has_update_action(update_query)
         self.assertIsNone(result)
 
-        triple = self.se.res, ProvEntity.iri_has_update_query, Literal(update_query)
+        triple = self.se.res, ProvEntity.iri_has_update_query, Literal(update_query, datatype=XSD.string)
         self.assertIn(triple, self.se.g)
 
     def test_has_description(self):
@@ -91,7 +91,7 @@ def test_has_description(self):
         result = self.se.has_description(description)
         self.assertIsNone(result)
 
-        triple = self.se.res, ProvEntity.iri_description, Literal(description)
+        triple = self.se.res, ProvEntity.iri_description, Literal(description, datatype=XSD.string)
         self.assertIn(triple, self.se.g)
 
     def test_has_resp_agent(self):
diff --git a/oc_ocdm/test/prov/prov_counter.db b/oc_ocdm/test/prov/prov_counter.db
index 8964e9d..cd8b1e0 100644
Binary files a/oc_ocdm/test/prov/prov_counter.db and b/oc_ocdm/test/prov/prov_counter.db differ
diff --git a/oc_ocdm/test/storer/test_storer.py b/oc_ocdm/test/storer/test_storer.py
index b786a0b..519af8f 100644
--- a/oc_ocdm/test/storer/test_storer.py
+++ b/oc_ocdm/test/storer/test_storer.py
@@ -110,7 +110,7 @@ def test_store_graphs_in_file(self):
                         <http://test/br/0601/prov/se/1> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/ns/prov#Entity> <http://test/br/0601/prov/> .
                         <http://test/br/0601/prov/se/1> <http://www.w3.org/ns/prov#specializationOf> <http://test/br/0601> <http://test/br/0601/prov/> .
                         <http://test/br/0601/prov/se/1> <http://www.w3.org/ns/prov#wasAttributedTo> <http://resp_agent.test/> <http://test/br/0601/prov/> .
-                        <http://test/br/0601/prov/se/1> <http://purl.org/dc/terms/description> "The entity 'http://test/br/0601' has been created." <http://test/br/0601/prov/> .
+                        <http://test/br/0601/prov/se/1> <http://purl.org/dc/terms/description> "The entity 'http://test/br/0601' has been created."^^<http://www.w3.org/2001/XMLSchema#string> <http://test/br/0601/prov/> .
                     """, format="nquads")
                     for s, p, o, c in data_g.quads():
                         if p == URIRef("http://www.w3.org/ns/prov#generatedAtTime"):
@@ -133,7 +133,7 @@ def test_store_graphs_in_file(self):
                 <http://test/br/0601/prov/se/1> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/ns/prov#Entity> <http://test/br/0601/prov/> .
                 <http://test/br/0601/prov/se/1> <http://www.w3.org/ns/prov#specializationOf> <http://test/br/0601> <http://test/br/0601/prov/> .
                 <http://test/br/0601/prov/se/1> <http://www.w3.org/ns/prov#wasAttributedTo> <http://resp_agent.test/> <http://test/br/0601/prov/> .
-                <http://test/br/0601/prov/se/1> <http://purl.org/dc/terms/description> "The entity 'http://test/br/0601' has been created." <http://test/br/0601/prov/> .
+                <http://test/br/0601/prov/se/1> <http://purl.org/dc/terms/description> "The entity 'http://test/br/0601' has been created."^^<http://www.w3.org/2001/XMLSchema#string> <http://test/br/0601/prov/> .
             """, format="nquads")
             for s, p, o, c in prov_unzipped.quads():
                 if p == URIRef("http://www.w3.org/ns/prov#generatedAtTime"):
diff --git a/pyproject.toml b/pyproject.toml
index c055ac4..f478817 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "oc_ocdm"
-version = "9.1.0"
+version = "9.1.1"
 description = "Object mapping library for manipulating RDF graphs that are compliant with the OpenCitations datamodel."
 authors = [
     "Silvio Peroni <essepuntato@gmail.com>",