1 files changed, 6 insertions, 1 deletions
diff --git a/src/class.c b/src/class.c
index b5c561d06..df034737e 100644
--- a/src/class.c
+++ b/src/class.c
@@ -489,14 +489,19 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         mrb_value ss;
         struct RString *s;
         char **ps;
+        mrb_int len;
 
         ps = va_arg(ap, char**);
         if (i < argc) {
           ss = to_str(mrb, *sp++);
           s = mrb_str_ptr(ss);
-          if ((mrb_int)strlen(s->ptr) < s->len) {
+          len = (mrb_int)strlen(s->ptr);
+          if (len < s->len) {
             mrb_raise(mrb, E_ARGUMENT_ERROR, "String contains NUL");
           }
+          else if (len > s->len) {
+            mrb_str_modify(mrb, s);
+          }
           *ps = s->ptr;
           i++;
         }