diff --git a/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java b/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java
index e9a2f2a..6f0cbba 100644
--- a/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java
+++ b/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java
@@ -129,7 +129,7 @@
 
         @Override
         public String toString() {
-            return "NULL";
+            return "null";
         }
 
         @Override
@@ -166,7 +166,7 @@
 
         @Override
         public String toString() {
-            return "MISSING";
+            return "missing";
         }
 
         @Override