diff --git a/strbuf.c b/strbuf.c
index 3e04addc22..8610965d53 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -106,12 +106,10 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
 void strbuf_grow(struct strbuf *sb, size_t extra)
 {
 	int new_buf = !sb->alloc;
-	if (unsigned_add_overflows(extra, 1) ||
-	    unsigned_add_overflows(sb->len, extra + 1))
-		die("you want to use way too much memory");
+	size_t new_len = st_add3(sb->len, extra, 1);
 	if (new_buf)
 		sb->buf = NULL;
-	ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
+	ALLOC_GROW(sb->buf, new_len, sb->alloc);
 	if (new_buf)
 		sb->buf[0] = '\0';
 }