C = QQ[x]
A = C[y]
B = C[z]
f = map(B,A,{z})
matB = matrix{{1_B}}
kernel map(B^1, A^1, f, matB)


This happens on the trunk, but not in 1.4:

i8 : kernel map(B^1, A^1, f, matB)
../../M2/trunk/M2/Macaulay2/m2/matrix3.m2:45:14:(1):[5]: error: assertion failed
../../M2/trunk/M2/Macaulay2/m2/matrix3.m2:45:14:(1):[5]: --entering debugger (type help to see debugger commands)
../../M2/trunk/M2/Macaulay2/m2/matrix3.m2:45:14-45:52: --source code:
             assert( degreesRing R === degreesRing G );
