diff --git a/bootstraptest/test_knownbug.rb b/bootstraptest/test_knownbug.rb
index 8ae5dc741c..23a020a771 100644
--- a/bootstraptest/test_knownbug.rb
+++ b/bootstraptest/test_knownbug.rb
@@ -42,3 +42,6 @@ assert_normal_exit %q{
   }
 }, '[ruby-dev:31985]'
 
+assert_normal_exit %q{
+  Regexp.union("a", "a")
+}