--- old/src/share/vm/gc_implementation/g1/concurrentMark.inline.hpp	2014-08-05 14:39:25.067073623 +0200
+++ new/src/share/vm/gc_implementation/g1/concurrentMark.inline.hpp	2014-08-05 14:39:24.850527224 +0200
@@ -323,7 +323,7 @@
   ++_refs_reached;
 
   HeapWord* objAddr = (HeapWord*) obj;
-  assert(obj->is_oop_or_null(true /* ignore mark word */), "Error");
+  assert(obj->is_oop_or_null(true /* ignore mark word */), err_msg("expected an oop or NULL: " PTR_FORMAT, p2i(obj)));
   if (_g1h->is_in_g1_reserved(objAddr)) {
     assert(obj != NULL, "null check is implicit");
     if (!_nextMarkBitMap->isMarked(objAddr)) {