title: TypeError: Attempted to assign to readonly property - when typing in a text field
body: <p>I recently updated my Angular from 1.5.x to 1.6.4 and now, when I go to a form, I get the below error message whenever I try to type something up in the form/textbox:</p>
  
  <pre><code>TypeError: Attempted to assign to readonly property.
  </code></pre>
  
  <p>This is my controller:</p>
  
  <pre><code>mainApp.controller('newPostController', ['$scope', '$http', function($scope, $http){
  
  $scope.post = '';
  $scope.postCreated = false;
  
  $scope.makeNewPost = function() {
  
      $http.post('/api/post', {
          title:          $scope.post.title,
      })
      .then(function(res){
          $scope.postCreated = true;
          //extra code not related to the form itself...
    };
  }]);
  </code></pre>
  
  <p>My HTML looks like this:</p>
  
  <pre><code>&lt;form ng-submit="makeNewPost()"&gt;
  &lt;div class="form-group"&gt;
      &lt;label for="title" class="control-label"&gt;Title&lt;/label&gt;
      &lt;input type="text" autocomplete="off" class="form-control" ng-model="post.title" id="title" required="required"&gt;
  &lt;/div&gt;
  
  &lt;input type="submit" value="Submit"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>I looked this error up and everything I am seeing has nothing to do with what my set up is like.</p>
  
  <p>Please help me out on this. I don't know where to go from here.</p>
  
  <p>Thanks</p>