1 files changed, 1 insertions, 0 deletions
diff --git a/string.c b/string.c
index 9982e561b7..7d7eb5c769 100644
--- a/string.c
+++ b/string.c
@@ -1208,6 +1208,7 @@ rb_str_insert(str, idx, str2)
 {
     long pos = NUM2LONG(idx);
 
+    rb_str_modify(str);
     if (pos == -1) {
 	pos = RSTRING(str)->len;
     }