Passing dictionary for receiver_tensor in tf.estimator.export.ServingInputReceiver

I am trying to use the following json_serving_fn for predict on my tf model.

def json_serving_input_fn():
  """Build the serving inputs."""
  feature_placeholders = {'a': tf.placeholder(shape=[None], dtype='string'),
            'b': tf.placeholder(shape=[None], dtype='string'), 
            'c': tf.placeholder(shape=[1,10], dtype='string')}
  return tf.estimator.export.ServingInputReceiver(
    features=feature_placeholders,
      receiver_tensors=feature_placeholders)

The train and evaluation processes complete as intended (and orthogonal to the issue), but my serving function fails with the following error code:

INFO:tensorflow:Signatures EXCLUDED from export because they cannot be be served via TensorFlow Serving APIs:                                                                         
INFO:tensorflow:'serving_default' : Classification input must be a single string Tensor; got {'symptoms': <tf.Tensor 'Placeholder_2:0' shape=(1,10) dtype=string>, 'age': <tf.Tensor 'Placeholder_1:0' shape=(?,) dtype=string>, 'sex': <tf.Tensor 'Placeholder:0' shape=(?,) dtype=string>}     

I intend to pass the following json for prediction:

{"a":"x","b":"y","c":["p1","p2","","","","","","","",""]}

How do I rewrite my json_serving_input_fun to accomplish that?