RecurrentModel without Cell

[mlUser02]

Good evening,
I was looking upon your flawless package and stumbled over an issue I am not able to solute.

I am trying to include the usual Keras layers in a RecurrentModel without using your RecurrentCells (in all the examples these are used additionally). A miniature example could look like this:

inp = Input(shape=(1,))
inp_refeed = Input(shape=(1,))

inp_intern = add([inp, inp_refeed])
out = Dense(1)(inp_intern)


rnn = RecurrentModel(input=inp, output=out, readout_input=inp_refeed, teacher_force=True)

out = rnn(inputs=inp, ground_truth = inp_refeed)

but recurrentshop complains about

File "recurrentshop_without_cell.py", line 36, in <module>
    out = rnn(inputs=inp, ground_truth = inp_refeed)
File "/.../site-packages/recurrentshop-1.0.0-py3.6.egg/recurrentshop/engine.py", line 473, in __call__
AssertionError: Required 3 inputs, received 4.

This is driving me crazy since I cannot find the error in my thoughts.
Thank you very much in advance!