### What is the meaning of string index out of range?

I am dealing with [35,6] linear code over finite filed with 2 elements. I am able to get an output of its generator matrix as 6x35 matrix, but while computing parity check matrix which will be of size 29x35, I am getting following output:
IndexError:string index out of range

Does it mean that its size is two large? If still it is so, how to get that matrix in sage??