title: is_valid() throw exception when a simple form is invalid in django 1.6
body: <p>When the submitted Form is valid there is no problem but when the submitted Form is invalid(In our case It is blank for example) then the is_valid() throw this exception:
  (as I know is_valid() must return false in this situation)</p>
  
  <pre><code>Environment:
  
  
  Request Method: POST
  Request URL: http://127.0.0.1:8000/home/sign-up
  
  Django Version: 1.6
  Python Version: 3.3.2
  Installed Applications:
  ('django.contrib.admin',
   'django.contrib.auth',
   'django.contrib.contenttypes',
   'django.contrib.sessions',
   'django.contrib.messages',
   'django.contrib.staticfiles',
   'employer',
   'employee',
   'home',
   'admin',
   'django.contrib.admin')
  Installed Middleware:
  ('django.contrib.sessions.middleware.SessionMiddleware',
   'django.middleware.common.CommonMiddleware',
   'django.middleware.csrf.CsrfViewMiddleware',
   'django.contrib.auth.middleware.AuthenticationMiddleware',
   'django.contrib.messages.middleware.MessageMiddleware',
   'django.middleware.clickjacking.XFrameOptionsMiddleware')
  
  
  Traceback:
  File "C:\Python33\lib\site-packages\django\core\handlers\base.py" in get_response
    114.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\Sina-PC\PycharmProjects\ـ\home\views.py" in sign_up
    32.         if  myForm.is_valid():
  File "C:\Python33\lib\site-packages\django\forms\forms.py" in is_valid
    129.         return self.is_bound and not bool(self.errors)
  File "C:\Python33\lib\site-packages\django\forms\forms.py" in errors
    121.             self.full_clean()
  File "C:\Python33\lib\site-packages\django\forms\forms.py" in full_clean
    273.         self._clean_fields()
  File "C:\Python33\lib\site-packages\django\forms\forms.py" in _clean_fields
    288.                     value = field.clean(value)
  File "C:\Python33\lib\site-packages\django\forms\fields.py" in clean
    149.         self.validate(value)
  File "C:\Python33\lib\site-packages\django\forms\fields.py" in validate
    125.             raise ValidationError(self.error_messages['required'], code='required')
  
  Exception Type: TypeError at /home/sign-up
  Exception Value: string indices must be integers
  </code></pre>
  
  <p>my form is :</p>
  
  <pre><code> __author__ = 'Sina-PC'
  
   from django import  forms
  
   class SignupForm(forms.Form):
       username=forms.CharField(max_length=10,widget=forms.TextInput);
       username.label='نام کاربری';
       username.required=True;
       username.error_messages="kkkkkk";
  </code></pre>
  
  <p>my view function is :</p>
  
  <pre><code>    def sign_up(request):
  
          if (request.method=="POST"):
              myForm=SignupForm(request.POST);
              if  myForm.is_valid():
                  print("do");
                  return  render(request,'home/signup-success.html',{'form':myForm});
  
          myForm=SignupForm();
          return  render(request,'home/sign-up.html',{'form':myForm});
  </code></pre>
  
  <p>and my template is :</p>
  
  <pre><code> &lt;form method="POST" action="{% url 'sign-up' %}"&gt;
                          {% csrf_token %}
                          &lt;div id="form1" &gt;
  
                          {{ form.as_p }}
  
                           &lt;/div&gt;
                              &lt;button type="submit" class="btn btn-primary pull-left " style="font-family: tahoma" id="registerBt"   &gt;
  مرحله بعد
                              &lt;/button&gt;
                          &lt;div class="clearfix"&gt;&lt;/div&gt;
                      &lt;/form&gt;
  </code></pre>
  
  <p>EDIT:Full trace back added.</p>