1 files changed, 1 insertions, 0 deletions
diff --git a/src/string.c b/src/string.c
index 1c54ffa72..2b1a27fee 100644
--- a/src/string.c
+++ b/src/string.c
@@ -781,6 +781,7 @@ num_index:
         }
       }
     default:
+      if (mrb_nil_p(indx)) mrb_raise(mrb, E_TYPE_ERROR, "can't convert nil into Integer");
       idx = mrb_fixnum(indx);
       goto num_index;
   }