diff --git a/shared/strbuf.c b/shared/strbuf.c
index 172558a..ef99eb7 100644
--- a/shared/strbuf.c
+++ b/shared/strbuf.c
@@ -36,12 +36,12 @@ static bool buf_realloc(struct strbuf *buf, size_t sz)
 
 static bool strbuf_reserve_extra(struct strbuf *buf, size_t n)
 {
+	if (n < buf->size - buf->used)
+		return true;
+
 	if (uaddsz_overflow(buf->used, n, &n) || n >= SIZE_MAX - BUF_STEP)
 		return false;
 
-	if (n < buf->size)
-		return true;
-
 	if (++n % BUF_STEP)
 		n = ((n / BUF_STEP) + 1) * BUF_STEP;