MinibatchLoader.lua:442: bad argument #1 to 'ipairs' (table expected, got nil)

[WenkangHu]

When run make train_content . I receive this error:

loading data from text file dat/qb-all/content_data.txt
loading dat/qb-all/content_data.txt.mapping.t7
maximum sequence length: 266
vocab size: 22052
# answers/classes: 1046
# users: 0
# categories: 54
creating mappings: 0.22
loading dat/qb-all/content_data.txt.tensor.t7
/home/ethereal/torch/install/bin/luajit: ...aduationproject/opponent-master/util/MinibatchLoader.lua:442: bad argument #1 to 'ipairs' (table expected, got nil)
stack traceback:
        [C]: in function 'ipairs'
        ...aduationproject/opponent-master/util/MinibatchLoader.lua:442: in function 'text_to_tensor'
        ...aduationproject/opponent-master/util/MinibatchLoader.lua:21: in function 'load_data'
        train_content.lua:106: in main chunk
        [C]: in function 'dofile'
        ...real/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:150: in main chunk
        [C]: at 0x561f089bb570
Makefile:60: recipe for target 'train_content' failed
make: *** [train_content] Error 1

How can I solve this problem?