diff --git a/drd/drd_malloc_wrappers.c b/drd/drd_malloc_wrappers.c
index 3dcbb44..d380ecc 100644
--- a/drd/drd_malloc_wrappers.c
+++ b/drd/drd_malloc_wrappers.c
@@ -228,7 +228,8 @@
          
          /* Free old memory. */
          VG_(cli_free)(p_old);
-         s_stop_using_mem_callback(mc->data, mc->size);
+         if (mc->size > 0)
+            s_stop_using_mem_callback(mc->data, mc->size);
          VG_(HT_remove)(s_malloc_list, (UWord)p_old);
 
          /* Update state information. */