diff --git a/string.c b/string.c
index 6fd5866336..f556cdf180 100644
--- a/string.c
+++ b/string.c
@@ -2473,6 +2473,7 @@ rb_str_modify_expand(VALUE str, long expand)
     else if (expand > 0) {
         RESIZE_CAPA_TERM(str, len + expand, termlen);
     }
+    ENC_CODERANGE_CLEAR(str);
 }
 
 /* As rb_str_modify(), but don't clear coderange */