Index: src/main/groovy/lang/MetaClassImpl.java
===================================================================
--- src/main/groovy/lang/MetaClassImpl.java	(revision 19149)
+++ src/main/groovy/lang/MetaClassImpl.java	(working copy)
@@ -2280,6 +2280,11 @@
             return;
         }
 
+        if (name == null && !isStatic && this.isMap) {
+            ((Map) object).put(name, newValue);
+            return;
+        }
+        
         //----------------------------------------------------------------------
         // Unwrap wrapped values fo now - the new MOP will handle them properly
         //----------------------------------------------------------------------
Index: src/test/groovy/bugs/Groovy4029Bug.groovy
===================================================================
--- src/test/groovy/bugs/Groovy4029Bug.groovy	(revision 0)
+++ src/test/groovy/bugs/Groovy4029Bug.groovy	(revision 0)
@@ -0,0 +1,9 @@
+package groovy.bugs
+
+class Groovy4029Bug extends GroovyTestCase {
+    void testAddNullKeyEntryInMapUsingSubscriptNotation() {
+		Map m = [:]
+		m[null] = null
+		assert m.size() == 1
+    }
+}