diff --git a/test/-ext-/string/test_fstring.rb b/test/-ext-/string/test_fstring.rb
index 0514814dd4..1b3b15c922 100644
--- a/test/-ext-/string/test_fstring.rb
+++ b/test/-ext-/string/test_fstring.rb
@@ -60,7 +60,7 @@ class Test_String_Fstring < Test::Unit::TestCase
   def test_singleton_class
     str = noninterned_name.force_encoding("us-ascii")
     fstr = Bug::String.fstring(str)
-    assert_raise(RuntimeError) {fstr.singleton_class}
+    assert_raise(TypeError) {fstr.singleton_class}
   end
 
   class S < String