diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index a5eb0a5844..e9c7ee62b2 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -338,6 +338,10 @@ class TestProcess < Test::Unit::TestCase
     ensure
       ENV["hmm"] = old
     end
+
+    assert_raise_with_message(ArgumentError, /fo=fo/) {
+      system({"fo=fo"=>"ha"}, *ENVCOMMAND)
+    }
   end
 
   def test_execopt_env_path