diff --git a/shallow.c b/shallow.c
index 7d5ea0cd39..4c4486ad67 100644
--- a/shallow.c
+++ b/shallow.c
@@ -368,7 +368,7 @@ static uint32_t *paint_alloc(struct paint_info *info)
 	unsigned nr = (info->nr_bits + 31) / 32;
 	unsigned size = nr * sizeof(uint32_t);
 	void *p;
-	if (!info->pool_count || info->free + size > info->end) {
+	if (!info->pool_count || size > info->end - info->free) {
 		if (size > POOL_SIZE)
 			die("BUG: pool size too small for %d in paint_alloc()",
 			    size);