diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 7b06c6ab5..0717e44c8 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,7 +1,12 @@
 ## Validator Changes
 
-* no changes
+* work around THO 6.0.0 problem (hack, to be reversed later)
+* Improve MeasureReport validation for checking subject count
+* Fix for R2B Resource.id cardinality problem
+* Fix for validator using wrong property for list determination when parsing json
 
 ## Other code changes
 
-* no changes
\ No newline at end of file
+* no changes
+
+
diff --git a/org.hl7.fhir.r4/src/main/java/org/hl7/fhir/r4/model/NamingSystem.java b/org.hl7.fhir.r4/src/main/java/org/hl7/fhir/r4/model/NamingSystem.java
index ce5c67af6..5313ee7a2 100644
--- a/org.hl7.fhir.r4/src/main/java/org/hl7/fhir/r4/model/NamingSystem.java
+++ b/org.hl7.fhir.r4/src/main/java/org/hl7/fhir/r4/model/NamingSystem.java
@@ -347,6 +347,8 @@ public class NamingSystem extends MetadataResource {
     }
 
     public String toCode(NamingSystemIdentifierType code) {
+      if (code == NamingSystemIdentifierType.NULL)
+        return null;
       if (code == NamingSystemIdentifierType.OID)
         return "oid";
       if (code == NamingSystemIdentifierType.UUID)
diff --git a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/model/NamingSystem.java b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/model/NamingSystem.java
index 6639afdd5..80aec8825 100644
--- a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/model/NamingSystem.java
+++ b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/model/NamingSystem.java
@@ -312,6 +312,8 @@ public class NamingSystem extends MetadataResource {
         throw new FHIRException("Unknown NamingSystemType code '"+codeString+"'");
         }
     public String toCode(NamingSystemType code) {
+      if (code == NamingSystemType.NULL)
+        return null;
       if (code == NamingSystemType.CODESYSTEM)
         return "codesystem";
       if (code == NamingSystemType.IDENTIFIER)