table is nil error

[j-titus]

using "th create_dataset.lua -train /Users/mac/USNerve/train/images -trainOutput /Users/j-titus/USNerve/train/images.h5 -test /Users/j-titus/USNerve/test -testOutput /Users/j-titus/USNerve/test.h5"

throws the error
/Users/j-titus/torch/install/bin/luajit: create_dataset.lua:82: table index is nil
stack traceback:
create_dataset.lua:82: in function 'create_train_h5'
create_dataset.lua:118: in main chunk
[C]: in function 'dofile'
.../j-titus/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:150: in main chunk
[C]: at 0x010f836a10

is it a path error or have I missed something? All of my train and mask png files are in the same folder

[j-titus]

the masks are in the [train path]/masks