nested = [[1, 2, 3, 4, 5, 6],[1,2,3],[]]
nestedtest2 = [[1, 2, 3], [1, 2, 3], [1, 2   ]] # <--- This works

def column_sums(nested):
    summary = [ sum([nested[j][i] for j in range(len(nested[i]))]) for i in range(len(nested)) if len(nested[i]) > 0 ]

    return summary


if __name__ == '__main__':
    print(column_sums(nested)) # returns an index error when I call with input nested