diff --git a/plugins/proto-interface/src/main/java/org/polypheny/db/protointerface/utils/PolyValueSerializer.java b/plugins/proto-interface/src/main/java/org/polypheny/db/protointerface/utils/PolyValueSerializer.java
index 48c4be027a..0255480a60 100644
--- a/plugins/proto-interface/src/main/java/org/polypheny/db/protointerface/utils/PolyValueSerializer.java
+++ b/plugins/proto-interface/src/main/java/org/polypheny/db/protointerface/utils/PolyValueSerializer.java
@@ -82,7 +82,7 @@ public static ProtoEntry serializeToProtoEntry( Map.Entry<PolyValue, PolyValue>
 
 
     public static ProtoValue serialize( PolyValue polyValue ) {
-        if ( polyValue == null ) {
+        if ( polyValue == null || polyValue.isNull() ) {
             return serializeAsProtoNull();
         }
         switch ( polyValue.getType() ) {