keras: Tensorflow only: TypeError: Expected int32, got list containing Tensors of type '_Message' instead.

This occurs only when using TF:

graph = Graph()
graph.add_input(name='input', input_shape=(3, 40, 40))
graph.add_node(Convolution2D(32, 9, 9, activation='relu'), name='conv1', input='input')
graph.add_node(Convolution2D(32, 9, 9, activation='relu'), name='conv2', input='conv1')
graph.add_node(MaxPooling2D(pool_size=(2, 2)), name='pool', input='conv2')
graph.add_node(Dropout(0.25), name='drop', input='pool')
graph.add_node(Flatten(), name='flatten', input='drop')
graph.add_node(Dense(640, activation='relu'), name='ip', input='flatten')
graph.add_node(Dropout(0.5), name='drop_out', input='ip')
graph.add_node(Dense(19, activation='softmax'), name='result', input='drop_out')

I get: TypeError: Expected int32, got list containing Tensors of type '_Message' instead. Theano version works fine

Traceback (most recent call last):
  File ".py", line 38, in <module>
    graph.add_node(Flatten(), name='flatten', input='drop')
  File "/usr/local/lib/python2.7/dist-packages/keras/legacy/models.py", line 169, in add_node
    layer.add_inbound_node(self._graph_nodes[input])
  File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 569, in add_inbound_node
    Node.create_node(self, inbound_layers, node_indices, tensor_indices)
  File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 150, in create_node
    output_tensors = to_list(outbound_layer.call(input_tensors[0], mask=input_masks[0]))
  File "/usr/local/lib/python2.7/dist-packages/keras/layers/core.py", line 311, in call
    return K.batch_flatten(x)
  File "/usr/local/lib/python2.7/dist-packages/keras/backend/tensorflow_backend.py", line 776, in batch_flatten
    x = tf.reshape(x, [-1, prod(shape(x)[1:])])
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/gen_array_ops.py", line 1092, in reshape
    name=name)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/op_def_library.py", line 411, in apply_op
    as_ref=input_arg.is_ref)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/ops.py", line 566, in convert_to_tensor
    ret = conversion_func(value, dtype=dtype, name=name, as_ref=as_ref)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/constant_op.py", line 179, in _constant_tensor_conversion_function
    return constant(v, dtype=dtype, name=name)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/constant_op.py", line 162, in constant
    tensor_util.make_tensor_proto(value, dtype=dtype, shape=shape))
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/tensor_util.py", line 332, in make_tensor_proto
    _AssertCompatible(values, dtype)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/tensor_util.py", line 272, in _AssertCompatible
    (dtype.name, repr(mismatch), type(mismatch).__name__))
TypeError: Expected int32, got list containing Tensors of type '_Message' instead.