Lifting from fraction field failure

[pzinn]

this fails even though it shouldn't:

i1 : R=QQ[x];

i2 : F=frac R;

i3 : lift(1/123_F,R)
stdio:3:4:(3): error: cannot lift given ring element

i4 : lift(1/123_F,QQ)
stdio:4:4:(3): error: cannot lift given ring element

This can be traced back to frac.cpp:

bool FractionField::lift(const Ring *Rg,
                         const ring_elem f,
                         ring_elem &result) const
{
  // Rg = R ---> frac R
  // f is an element of frac R.

  ring_elem
      hdenom;  // used in the case when the denominator can be a unit, but not 1
               // e.g. when this = frac (QQ[x,y,z]).  Is an element of
  if (Rg == R_)
    {
      frac_elem *h = FRAC_VAL(f);
      if (R_->is_equal(h->denom, R_->one()))
        {
          result = R_->copy(h->numer);
          return true;
        }
      else
        {
          if (R_->is_field())
            {
              // XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
              // try to lift denominator.  If can, can lift, otherwise not.
              if (R_->lift(R_, h->denom, hdenom))
                {
                  ring_elem hinv = R_->invert(hdenom);
                  result = R_->mult(hinv, h->numer);
                  return true;
                }
            }
        }
    }
  return false;
}

I suspect the offending line is if (R_->is_field()) which is always false (if R_ is a field, then it's its own fraction field).
Removing it seems to fix the issue, though I'm not sure exactly what's going on. (it should probably test for invertibility of the denominator)