diff --git a/applications/NXcanSAS.nxdl.xml b/applications/NXcanSAS.nxdl.xml
index fb3d0ae6c1..e562257c4d 100644
--- a/applications/NXcanSAS.nxdl.xml
+++ b/applications/NXcanSAS.nxdl.xml
@@ -937,7 +937,7 @@
           Description of the radiation source.
         </doc>
         
-        <field name="radiation" maxOccurs="1"
+        <field name="radiation" maxOccurs="1" optional="true"
 			    deprecated="Use either (or both) ``probe`` or ``type`` fields from ``NXsource`` (issue #765)">
           <doc>
             Name of the radiation used.
diff --git a/contributed_definitions/NXsnsevent.nxdl.xml b/contributed_definitions/NXsnsevent.nxdl.xml
index 65f699d094..934ada4985 100644
--- a/contributed_definitions/NXsnsevent.nxdl.xml
+++ b/contributed_definitions/NXsnsevent.nxdl.xml
@@ -16,7 +16,7 @@
         </doc>
         <group type="NXlog" minOccurs="1">
           <field name="average_value" type="NX_FLOAT"/>
-          <field name="average_value_error" type="NX_FLOAT"
+          <field name="average_value_error" type="NX_FLOAT" optional="true"
             deprecated="see https://github.com/nexusformat/definitions/issues/821"/>
           <field name="average_value_errors" type="NX_FLOAT"/>
           <field name="description"/>
@@ -39,7 +39,7 @@
             Motor logs from cvinfo file.
           </doc>
           <field name="average_value" type="NX_FLOAT"/>
-          <field name="average_value_error" type="NX_FLOAT"
+          <field name="average_value_error" type="NX_FLOAT" optional="true"
             deprecated="see https://github.com/nexusformat/definitions/issues/821"/>
           <field name="average_value_errors" type="NX_FLOAT"/>
           <field name="description"/>
diff --git a/contributed_definitions/NXsnshisto.nxdl.xml b/contributed_definitions/NXsnshisto.nxdl.xml
index 27b1136f80..2e94b7c71e 100644
--- a/contributed_definitions/NXsnshisto.nxdl.xml
+++ b/contributed_definitions/NXsnshisto.nxdl.xml
@@ -16,7 +16,7 @@
         </doc>
         <group type="NXlog" minOccurs="1">
           <field name="average_value" type="NX_FLOAT"/>
-          <field name="average_value_error" type="NX_FLOAT"
+          <field name="average_value_error" type="NX_FLOAT" optional="true"
                  deprecated="see https://github.com/nexusformat/definitions/issues/821"/>
           <field name="average_value_errors" type="NX_FLOAT"/>
           <field name="description"/>
@@ -39,7 +39,7 @@
             Motor logs from cvinfo file.
           </doc>
           <field name="average_value" type="NX_FLOAT"/>
-          <field name="average_value_error" type="NX_FLOAT"
+          <field name="average_value_error" type="NX_FLOAT" optional="true"
                  deprecated="see https://github.com/nexusformat/definitions/issues/821"/>
           <field name="average_value_errors" type="NX_FLOAT"/>
           <field name="description"/>