פירוש על קידושין 119:11

In the end, the Talmud admits that while it is preferable for her to have halitzah in a case where he says “from today and after my death” if she does have yibbum it is not a problem. Again, in this case there is no real doubt to R. Yohanan that this is not a valid divorce. It is only a rabbinic prohibition that she should have halitzah and not yibbum lest people allow the woman divorced by a man who said “from today if I die” who most certainly may not have yibbum.