diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index a98f1a060c..536ec21f4f 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -245,7 +245,7 @@ func (e *MarshalerError) Error() string {
 // Unwrap returns the underlying error.
 func (e *MarshalerError) Unwrap() error { return e.Err }
 
-var hex = "0123456789abcdef"
+const hex = "0123456789abcdef"
 
 // An encodeState encodes JSON into a bytes.Buffer.
 type encodeState struct {