diff --git a/main.c b/main.c
index 60be449..3821143 100644
--- a/main.c
+++ b/main.c
@@ -100,7 +100,7 @@ void free(void* ptr)
 	if (!ptr)
 		return;
 
-	struct block* b = ptr - 1;
+	struct block* b = (struct block*)ptr - 1;
 	b->free = 1;
 
 	struct block* link = b->next;