title: AttributeError: 'generator' object has no attribute 'append'
body: <p>I used to run this code as a unittest but suddenly it's not working</p>
  <p>code:</p>
  <pre><code>    import requests
      import pytest
    
      class TestSample:
          url = 'https://something/apiname'
          @pytest.fixture()
          def post_request(self, data):
              url = self.url + '/1'
              res = requests.post(url, headers={&quot;Content-Type&quot;: &quot;application/json&quot;},json=data)
              yield res
      
          @pytest.mark.parametrize(&quot;data,result&quot;,
                      [( {&quot;ids&quot;: [&quot;524234&quot;, &quot;12341&quot;, &quot;97555&quot;]},200))]
          def test_post_sample_data(self, data, result, post_request):
               rs = post_request
               assert rs.status_code == result
  </code></pre>
  <p>Error:</p>
  <pre><code>platform win32 -- Python 3.6.4, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
  plugins: hypothesis-5.37.4, cases-2.3.0
  test_selectsample.py:None (test_selectsample.py)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pluggy\hooks.py:286: in __call__
      return self._hookexec(self, self.get_hookimpls(), kwargs)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pluggy\manager.py:93: in _hookexec
      return self._inner_hookexec(hook, methods, kwargs)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pluggy\manager.py:87: in &lt;lambda&gt;
      firstresult=hook.spec.opts.get(&quot;firstresult&quot;) if hook.spec else False,
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\_pytest\python.py:249: in pytest_pycollect_makeitem
      initialnames, node, ignore_args=self._get_direct_parametrize_args(node)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pytest_cases\plugin.py:614: in getfixtureclosure
      _init_fixnames, super_closure, arg2fixturedefs = create_super_closure(fm, parentnode, fixturenames, ignore_args)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pytest_cases\plugin.py:668: in create_super_closure
      _merge(sorted_fixturenames, _init_fixnames)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pytest_cases\plugin.py:655: in _merge
      into_list.append(item)
  E   AttributeError: 'generator' object has no attribute 'append'
  collected 0 items / 1 error
  </code></pre>
  <p>=================================== ERRORS ====================================</p>
  <pre><code>_____________________ ERROR collecting test_selectids.py ______________________
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pluggy\hooks.py:286: in __call__
      return self._hookexec(self, self.get_hookimpls(), kwargs)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pluggy\manager.py:93: in _hookexec
      return self._inner_hookexec(hook, methods, kwargs)
      res = list(collector._genfunctions(name, obj))
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\_pytest\python.py:458: in _genfunctions
      initialnames, node, ignore_args=self._get_direct_parametrize_args(node)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pytest_cases\plugin.py:614: in getfixtureclosure
      _init_fixnames, super_closure, arg2fixturedefs = create_super_closure(fm, parentnode, fixturenames, ignore_args)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pytest_cases\plugin.py:668: in create_super_closure
      _merge(sorted_fixturenames, _init_fixnames)
  ..\..\AppData\Local\Programs\Python\Python36\lib\site-packages\pytest_cases\plugin.py:655: in _merge
      into_list.append(item)
  E   AttributeError: 'generator' object has no attribute 'append'
  =========================== short test summary info ===========================
  ERROR test_selectsample.py::TestSample - AttributeError: 'generator' object ha...
  !!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
  ============================== 1 error in 0.35s ===============================
  Process finished with exit code 0
  Assertion failed
  Assertion failed
  </code></pre>
  <p>Any idea what may be causing that? How can I prevent it?
  Thanks in advance</p>