diff --git a/src/main/org/apache/tools/ant/types/PropertySet.java b/src/main/org/apache/tools/ant/types/PropertySet.java
index 38cd262d0..23fcec50f 100644
--- a/src/main/org/apache/tools/ant/types/PropertySet.java
+++ b/src/main/org/apache/tools/ant/types/PropertySet.java
@@ -76,7 +76,7 @@ public class PropertySet extends DataType {
 
         public void setBuiltin(BuiltinPropertySetName b) {
             String built_in = b.getValue();
-            assertValid("builtin", builtin);
+            assertValid("builtin", built_in);
             this.builtin = built_in;
         }