TypeError: Outputs must be theano Variable or Out instances.

In custom keras layers, I am trying to get a layer's output which is returning three different tensors in a list.
below is the code:
test_fn = K.function([model.get_layer('sentence_input').input, model.get_layer('sentence2_input').input,
model.get_layer('sentence3_input').input, K.learning_phase()],
[model.get_layer('as_weights').output, model.get_layer('p_t').output,
model.get_layer('as_emb').output])

model.get_layer('as_emb').output is the layer I am getting the error message.

The objective is I want to pass a layer to theano.function's output attribute, which returns multiple tensors in a list. How to do this without the above error?

I am very much happy if someone helps me out in this regard.

Thanks.