title: Cannot interpret feed_dict key as Tensor
body: <p>I have a list of placeholders called "enqueue_ops" and a list of methods called "feed_fns", each of which returns a feed_dict. </p>
  
  <p>The queue runner of my graph is defined as:</p>
  
  <pre><code>queue_runner = feeding_queue_runner.FeedingQueueRunner(
              queue=queue, enqueue_ops=enqueue_ops,
              feed_fns=feed_fns)
  </code></pre>
  
  <p>However I got an error of </p>
  
  <pre><code>TypeError: Cannot interpret feed_dict key as Tensor: The name 'face_detection/x1' refers to an Operation, not a Tensor. Tensor names must be of the form "&lt;op_name&gt;:&lt;output_index&gt;".
  </code></pre>
  
  <p>But why are they looking at my feed_dict keys, while my feed_dict values are tensors that they don't want to look at? </p>
  
  <p>Thanks!!!</p>