cases(...).subs(...) should not subs unmatched expressions

[rwst]

sage: cases(((m == -1, log(x)), (m != -1, x^(m + 1)/(m + 1))))
cases(((m == -1, log(x)), (m != -1, x^(m + 1)/(m + 1))))
sage: _.subs(m==-1)
...
ValueError: power::eval(): division by zero

While subs should substitute matched expressions, e.g., the above with .subs(m==1) should return 1/2*x^2, unmatched expressions in the list should not be touched.