Question: Why do i get "Error, invalid subscript selector"??

I have my maple code as follows. I don't see why I got "Error, invalid subscript selector"? can anybody help me? Thanks in advance!!

>a := [2, 2, 3, 3, 3, 5];

b := [2, 2, 2, 3, 7];

for k to 10 do

  for j to 10 do

       c := b[k]; d := a[j];

if c = d then a := subsop(j = NULL, a);

    print(a);

     print(k);

    print(j);

     break

else print(k);

      print(j);

     print(-1)

end if

end do

end do;

print(a);
                             [2, 2, 3, 3, 3, 5]
                               [2, 2, 2, 3, 7]
                               [2, 3, 3, 3, 5]
                                      1
                                      1
                                [3, 3, 3, 5]
                                      2
                                      1
                                      3
                                      1
                                     -1
                                      3
                                      2
                                     -1
                                      3
                                      3
                                     -1
                                      3
                                      4
                                     -1
Error, invalid subscript selector
                                [3, 3, 3, 5]