kernel of a matrix defined over polynomial rings

edit

I have a matrix defined as a function of variables in a polynomial ring defined over finite field as follows-

              Gr.<xp,yp>=LaurentPolynomialRing(GF(2));
              M=Matrix(Gr,[[xp-1,0],
                                 [yp-1,0],
                                 [0,(yp^(-1))-1],
                                 [0,-(xp^(-1))+1]]);

I want to calculate the kernel of this matrix but the kernel function

M.kernel()

gives an error. What am I doing wrong?