diff --git a/src/util.h b/src/util.h
index 0dbf7f47101..fbfaf7aac1c 100644
--- a/src/util.h
+++ b/src/util.h
@@ -64,7 +64,7 @@ class Utf8Value {
       return str_;
     };
 
-    size_t length() {
+    size_t length() const {
       return length_;
     };