Error: bad argument #3 to '?' (torch.*Tensor expected, got nil)

[Alinahui]

Hi every one
i am getting this error, can anybody please help to resolve this issue:
Program 'lua' started in '/home/Alina/codes/captcha master' (pid: 7886).
tput: No value for $TERM and no -T specified
parsing file: dataset/ans.txt
parsing done
/opt/zbstudio/bin/linux/x64/lua: bad argument #3 to '?' (torch.*Tensor expected, got nil)
stack traceback:
[C]: at 0x7f9b6b7981b0
[C]: in function '__newindex'
./data.lua:43: in function 'loadY'
./data.lua:63: in function 'storeXY'
/home/Alina/codes/captcha master/captcha.lua:12: in main chunk
[C]: at 0x004054e8
Program completed in 0.08 seconds (pid: 7886).