diff --git a/asterixdb/asterix-om/src/main/java/org/apache/asterix/dataflow/data/common/TypeResolverUtil.java b/asterixdb/asterix-om/src/main/java/org/apache/asterix/dataflow/data/common/TypeResolverUtil.java
index 4f60c7f..415c1ae 100644
--- a/asterixdb/asterix-om/src/main/java/org/apache/asterix/dataflow/data/common/TypeResolverUtil.java
+++ b/asterixdb/asterix-om/src/main/java/org/apache/asterix/dataflow/data/common/TypeResolverUtil.java
@@ -102,7 +102,7 @@
 
         // Deals with the case one input type is null or missing.
         if (leftTypeTag == ATypeTag.MISSING || leftTypeTag == ATypeTag.NULL) {
-            return AUnionType.createUnknownableType(leftType);
+            return AUnionType.createUnknownableType(rightType);
         }
         if (rightTypeTag == ATypeTag.MISSING || rightTypeTag == ATypeTag.NULL) {
             return AUnionType.createUnknownableType(leftType);