diff --git a/strbuf.h b/strbuf.h
index fa07ecf094..4971743a24 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -105,6 +105,7 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s) {
 	strbuf_add(sb, s, strlen(s));
 }
 static inline void strbuf_addbuf(struct strbuf *sb, const struct strbuf *sb2) {
+	strbuf_grow(sb, sb2->len);
 	strbuf_add(sb, sb2->buf, sb2->len);
 }
 extern void strbuf_adddup(struct strbuf *sb, size_t pos, size_t len);