diff --git a/lmem.c b/lmem.c
index 77518707..53f8dcb9 100644
--- a/lmem.c
+++ b/lmem.c
@@ -108,7 +108,7 @@ l_noret luaM_toobig (lua_State *L) {
 */
 void luaM_free_ (lua_State *L, void *block, size_t osize) {
   global_State *g = G(L);
-  lua_assert((block == 0) == (block == NULL));
+  lua_assert((osize == 0) == (block == NULL));
   (*g->frealloc)(g->ud, block, osize, 0);
   g->GCdebt -= osize;
 }