diff --git a/lib/isc/mem.c b/lib/isc/mem.c
index 8fd0b1edbb..2a1346041e 100644
--- a/lib/isc/mem.c
+++ b/lib/isc/mem.c
@@ -15,7 +15,7 @@
  * PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: mem.c,v 1.157 2010/08/11 22:54:58 jinmei Exp $ */
+/* $Id: mem.c,v 1.158 2010/08/11 23:11:45 jinmei Exp $ */
 
 /*! \file */
 
@@ -1780,6 +1780,8 @@ ISC_MEMFUNC_SCOPE isc_boolean_t
 isc__mem_isovermem(isc_mem_t *ctx0) {
 	isc__mem_t *ctx = (isc__mem_t *)ctx0;
 
+	REQUIRE(VALID_CONTEXT(ctx));
+
 	/*
 	 * We don't bother to lock the context because 100% accuracy isn't
 	 * necessary (and even if we locked the context the returned value