The ring of M should be a base ring of R.
i1 : R = ZZ/101[x,y];
i2 : M = coker vars R o2 = cokernel {0} | x y | 1 o2 : R - module, quotient of R
i3 : M ** R[t] o3 = cokernel {0} | x y | 1 o3 : R[t] - module, quotient of (R[t])