diff --git a/nomenclature/error.py b/nomenclature/error.py
index 3ae87507..6549a908 100644
--- a/nomenclature/error.py
+++ b/nomenclature/error.py
@@ -39,7 +39,7 @@
     ),
     "RegionNotDefinedError": (
         "region_not_defined",
-        "Region(s) {regions} in {file} not found in RegionCodeList",
+        "Region(s)\n{regions}\nin {file}\nnot found in RegionCodeList",
     ),
 }
 
diff --git a/tests/test_region_aggregation.py b/tests/test_region_aggregation.py
index f10c7d89..bdeea947 100644
--- a/tests/test_region_aggregation.py
+++ b/tests/test_region_aggregation.py
@@ -144,9 +144,9 @@ def test_region_processor_not_defined(simple_definition):
     # definition
     error_msg = (
         "mappings.model_(a|b).*\n"
-        ".*region_a.*mapping_(1|2).yaml.*region_not_defined.*\n"
+        ".*\n.*region_a.*\n.*mapping_(1|2).yaml\n.*region_not_defined.*\n"
         "mappings.model_(a|b).*\n"
-        ".*region_a.*mapping_(1|2).yaml.*region_not_defined"
+        ".*\n.*region_a.*\n.*mapping_(1|2).yaml\n.*region_not_defined"
     )
 
     with pytest.raises(ValueError, match=error_msg):