diff --git a/vlib/builtin/map.v b/vlib/builtin/map.v
index 267880916c..9d92d1bd1c 100644
--- a/vlib/builtin/map.v
+++ b/vlib/builtin/map.v
@@ -167,7 +167,7 @@ fn preorder_keys(node &mapnode, keys mut []string, key_i int) int {
 	return i
 }
 
-pub fn (m mut map) keys() []string {
+pub fn (m &map) keys() []string {
 	mut keys := [''].repeat(m.size)
 	if isnil(m.root) {
 		return keys