diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 3bfb74d6a4..9006abde1b 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -992,6 +992,28 @@ void NormalizedMapCache::NormalizedMapCacheVerify() {
 }
 
 
+void Map::ZapInstanceDescriptors() {
+  DescriptorArray* descriptors = instance_descriptors();
+  if (descriptors == GetHeap()->empty_descriptor_array()) return;
+  FixedArray* contents = FixedArray::cast(
+      descriptors->get(DescriptorArray::kContentArrayIndex));
+  MemsetPointer(descriptors->data_start(),
+                GetHeap()->the_hole_value(),
+                descriptors->length());
+  MemsetPointer(contents->data_start(),
+                GetHeap()->the_hole_value(),
+                contents->length());
+}
+
+
+void Map::ZapPrototypeTransitions() {
+  FixedArray* proto_transitions = prototype_transitions();
+  MemsetPointer(proto_transitions->data_start(),
+                GetHeap()->the_hole_value(),
+                proto_transitions->length());
+}
+
+
 #endif  // DEBUG
 
 } }  // namespace v8::internal
diff --git a/src/objects-inl.h b/src/objects-inl.h
index eb1586ad0c..544443866e 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -3351,6 +3351,9 @@ void Map::clear_instance_descriptors() {
   Object* object = READ_FIELD(this,
                               kInstanceDescriptorsOrBitField3Offset);
   if (!object->IsSmi()) {
+#ifdef DEBUG
+    ZapInstanceDescriptors();
+#endif
     WRITE_FIELD(
         this,
         kInstanceDescriptorsOrBitField3Offset,
@@ -3376,6 +3379,11 @@ void Map::set_instance_descriptors(DescriptorArray* value,
     }
   }
   ASSERT(!is_shared());
+#ifdef DEBUG
+  if (value != instance_descriptors()) {
+    ZapInstanceDescriptors();
+  }
+#endif
   WRITE_FIELD(this, kInstanceDescriptorsOrBitField3Offset, value);
   CONDITIONAL_WRITE_BARRIER(
       heap, this, kInstanceDescriptorsOrBitField3Offset, value, mode);
@@ -3448,6 +3456,11 @@ void Map::set_prototype_transitions(FixedArray* value, WriteBarrierMode mode) {
   Heap* heap = GetHeap();
   ASSERT(value != heap->empty_fixed_array());
   value->set(kProtoTransitionBackPointerOffset, GetBackPointer());
+#ifdef DEBUG
+  if (value != prototype_transitions()) {
+    ZapPrototypeTransitions();
+  }
+#endif
   WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, value);
   CONDITIONAL_WRITE_BARRIER(
       heap, this, kPrototypeTransitionsOrBackPointerOffset, value, mode);
diff --git a/src/objects.h b/src/objects.h
index 22993f2568..358af4a9d4 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4855,6 +4855,14 @@ class Map: public HeapObject {
   Handle<Map> FindTransitionedMap(MapHandleList* candidates);
   Map* FindTransitionedMap(MapList* candidates);
 
+  // Zaps the contents of backing data structures in debug mode. Note that the
+  // heap verifier (i.e. VerifyMarkingVisitor) relies on zapping of objects
+  // holding weak references when incremental marking is used, because it also
+  // iterates over objects that are otherwise unreachable.
+#ifdef DEBUG
+  void ZapInstanceDescriptors();
+  void ZapPrototypeTransitions();
+#endif
 
   // Dispatched behavior.
 #ifdef OBJECT_PRINT