diff --git a/strbuf.c b/strbuf.c
index 8007be8fba..1df674e919 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -683,7 +683,7 @@ static void strbuf_add_urlencode(struct strbuf *sb, const char *s, size_t len,
 		    (!reserved && is_rfc3986_reserved(ch)))
 			strbuf_addch(sb, ch);
 		else
-			strbuf_addf(sb, "%%%02x", ch);
+			strbuf_addf(sb, "%%%02x", (unsigned char)ch);
 	}
 }