1 files changed, 9 insertions, 0 deletions
diff --git a/test/-ext-/string/test_fstring.rb b/test/-ext-/string/test_fstring.rb
index 39ed020892..0514814dd4 100644
--- a/test/-ext-/string/test_fstring.rb
+++ b/test/-ext-/string/test_fstring.rb
@@ -1,8 +1,11 @@
 # frozen_string_literal: false
 require 'test/unit'
 require '-test-/string'
+require_relative '../symbol/noninterned_name'
 
 class Test_String_Fstring < Test::Unit::TestCase
+  include Test_Symbol::NonInterned
+
   def assert_fstring(str)
     fstr = Bug::String.fstring(str)
     yield str
@@ -54,6 +57,12 @@ class Test_String_Fstring < Test::Unit::TestCase
     assert_fstring(str) {|s| assert_send([s, :respond_to?, :foo])}
   end
 
+  def test_singleton_class
+    str = noninterned_name.force_encoding("us-ascii")
+    fstr = Bug::String.fstring(str)
+    assert_raise(RuntimeError) {fstr.singleton_class}
+  end
+
   class S < String
   end