diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 2b79ace..33a2ab8 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1906,6 +1906,10 @@ def test_flag
       assert_raise(ArgumentError) do
         open(t.path, "rr") { }
       end
+
+      assert_raise(ArgumentError) do
+        open(t.path, "rbt") { }
+      end
     }
   end