פירוש על קידושין 123:13

In most of these cases the kiddushin is not effective because it is not in his power to free her himself or to cause her husband or sister to die or to cause the yavam to perform halitzah for her. However, right now the Talmud assumes that he has the power to convert. Therefore, he should, according to R. Yohanan, be able to say “behold, you are betrothed to me after I convert.”