String#[] should reject nil index

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;
   }

test/t/string.rb

@@ -72,6 +72,10 @@ assert('String#[]', '15.2.10.5.6') do
   assert_equal 'bc', e1
   assert_equal 'bc', a3
   assert_nil b3
+
+  assert_raise(TypeError) do
+    a[nil]
+  end
 end
 
 assert('String#[] with Range') do