Tensorflow ctc_loss_calculator: no valid path found

When running my neural network (bidirectional LSTMs) I am using the Conectionist Temporal Classification (CTC) for sound recognition. But at some point while coaching the Net, I get the following warning from Tensorflow almost every batch.

W tensorflow/core/util/ctc/ctc_loss_calculator.cc:144] No valid path found.

      

        
        
        
      

    

This leads to the loss of endless and broken workout.

Epoch 1/5000, train_cost = inf, train_ler = 3.891, valid_cost = inf, valid_ler = 5.433
Train decoding: 
Original: you want to go over and see his gang throw dirt
Decoded: hlvoyvrofuvulovowovwvoxvovoxwyzlwkngitewewewktwlwctnbkmpajxozovofovfvfwnsfvfnfavtieitstyvubeabmbmbjljaceutztqectpmgogovgvovjuvsvsihskikqlvnsmsmsmhmvwiecececeitmhmhfvrf tiet e gekesketksmvamnmamgmnm det ietutswsezvzovovjiecgs gs smsjs s g la ah kjrkmasanxrsdrhdrxgdhdaphxda th sxhsxrdsrsvr krs farsr rdrdakr lrsrsvrsrsrdrsrsraisdsrhrhdrajfrdhxrd d

      

        
        
        
      

    

What exactly does this mean and how can I solve this problem?