Index: src/main/org/codehaus/groovy/runtime/DefaultGroovyMethods.java
===================================================================
--- src/main/org/codehaus/groovy/runtime/DefaultGroovyMethods.java	(revision 19149)
+++ src/main/org/codehaus/groovy/runtime/DefaultGroovyMethods.java	(working copy)
@@ -4323,6 +4323,11 @@
         return value;
     }
 
+    public static <V> V putAt(Map<String,V> self, String key, V value) {
+        self.put(key, value);
+        return value;
+    }
+
     /**
      * Support the subscript operator for List
      * <pre class="groovyTestCase">assert [String, Long, Integer] == ["a",5L,2]["class"]</pre>
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
+    }
+}