diff --git a/shallow.c b/shallow.c
index b8fcfbef0f..b54244ffa9 100644
--- a/shallow.c
+++ b/shallow.c
@@ -534,7 +534,7 @@ static uint32_t *paint_alloc(struct paint_info *info)
 	unsigned nr = DIV_ROUND_UP(info->nr_bits, 32);
 	unsigned size = nr * sizeof(uint32_t);
 	void *p;
-	if (!info->pool_count || size > info->end - info->free) {
+	if (!info->pool_count || info->end < info->free + size) {
 		if (size > POOL_SIZE)
 			BUG("pool size too small for %d in paint_alloc()",
 			    size);