diff --git a/src/mem.c b/src/mem.c
index b928430628e0a73414cea6516934f2ddb2aa88cd..fc8df6f9b5964e94dc3114bc3a9b603cde4ad6e1 100644
--- a/src/mem.c
+++ b/src/mem.c
@@ -498,7 +498,7 @@ void rt_free(void *rmem)
 
 	/* ... which has to be in a used state ... */
 	RT_ASSERT(mem->used);
-	RT_ASSERT(mem->magic != HEAP_MAGIC);
+	RT_ASSERT(mem->magic == HEAP_MAGIC);
 	/* ... and is now unused. */
 	mem->used = 0;
 	mem->magic = 0;