I’ll check this on Monday and I post here. But some points to consider is
that I’m using both LSTM and GRU rnn, and with the bidirectional
architecture. And I remember that I first tested with -1, but this was not
working in my case, and the I used the solution above. I’ll confirm it in
the beginning of the week 
My prev_hidden is usually zero while training, I just have the parameter to
evaluate the model in some specific starting points.