Tweak the test

test/test_puma_server.rb

@@ -363,6 +363,17 @@ def test_lowlevel_error_message
     assert (data.size > 0), "Expected response message to be not empty"
   end
 
+  def test_lowlevel_error_handler_custom_response
+    options = { lowlevel_error_handler: ->(_err) { [200, {}, ["error page"]] } }
+    # setting the headers argument to nil will trigger exception inside Puma
+    broken_app = ->(_env) { [200, nil, []] }
+    server_run(**options, &broken_app)
+
+    data = send_http_and_read "GET / HTTP/1.0\r\n\r\n"
+
+    assert_match %r{HTTP/1.0 200 OK\r\nContent-Length: 10\r\n\r\nerror page}, data
+  end
+
   def test_force_shutdown_error_default
     server_run(force_shutdown_after: 2) do
       @server.stop
@@ -1362,15 +1373,4 @@ def test_rack_url_scheme_user
     data = send_http_and_read "GET / HTTP/1.0\r\n\r\n"
     assert_equal "user", data.split("\r\n").last
   end
-
-  # The server should send lowlevel_error handlers response to the client
-  def test_lowlevel_error_handler
-    options = { lowlevel_error_handler: ->(err) { [200, {}, ["error page"]] } }
-    app = ->(env) { [200, nil, []] }
-    server_run(**options, &app)
-
-    data = send_http_and_read "GET / HTTP/1.0\r\n\r\n"
-
-    assert_match /error page/, data
-  end
 end