error when predict.lua

[byuns9334]

Hi, I am trying to execute predict.lua on some toy text and wav file of Librispeech.

the toy text (test.txt) is : HE HOPED THERE WOULD BE STEW FOR DINNER TURNIPS AND CARROTS AND BRUISED POTATOES AND FAT MUTTON PIECES TO BE LADLED OUT IN THICK PEPPERED FLOUR FATTENED SAUCE

and when I do th Predict.lua -modelPath libri_deepspeech.t7.1 -audioPath prepare_datasets/LibriSpeech/test/test-clean/1089/134686/test.wav -dictionaryPath prepare_datasets/LibriSpeech/test/test-clean/1089/134686/test.txt ,

I get this error: /home/byuns9334/torch/install/bin/luajit: ./Mapper.lua:59: attempt to concatenate a nil value
stack traceback:
./Mapper.lua:59: in function 'tokensToText'
Predict.lua:44: in main chunk
[C]: in function 'dofile'
...9334/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:150: in main chunk
[C]: at 0x00405d50

how should I resolve this error?