diff --git a/src/util/string.c b/src/util/string.c
index d95ca18b3..44e6cd0aa 100644
--- a/src/util/string.c
+++ b/src/util/string.c
@@ -326,7 +326,7 @@ add_string_to_string(struct string *string, const struct string *from)
 	check_string_magic(string);
 	check_string_magic(from);
 
-	if (!*from->source) return NULL;
+	if (!from->length) return string; /* optimization only */
 
 	return add_bytes_to_string(string, from->source, from->length);
 }