What is the meaning of :IndexError:string index out of range [closed]

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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 too large? If still it is so, how to get that matrix in sage??