Error on crop: slice indices must be integers or None or have an __index__ method

Am getting "slice indices must be integers or None or have an __index__ method" if I try and use the `-c` option:

```
$ pdf-table-extract -i test.pdf -o test.txt -p 1
[works okay]
$ pdf-table-extract -i test.pdf -o test.txt -p 1 -c 0:0:3:3
slice indices must be integers or None or have an __index__ method
$ 
```