TT #656 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:05
When brothers dwell together and one of them dies and has no son, the wife of the dead shall not be outside to a strange man; her husband’s brother shall come upon her and take her to himself as a wife and perform the duty of a husband’s brother for her.
Torah Temimah points to Yevamot 20a.
TT lemma: לְאִישׁ זָר.
לְאִישׁ זָר – לְאִישׁ שֶׁהִיא זָרָה לוֹ, אָמְרָה תּוֹרָה לֹא תִהְיֶה לוֹ לְאִשָּׁה אֲפִלּוּ בִּמְקוֹם מִצְוָה.
English: “To a strange man” — to a man to whom she is a stranger (forbidden). The Torah says she shall not be his wife even in a context of mitzvah.
On the daf / in context: The verse excludes cases where the yevamah is otherwise forbidden to the yavam by a negative commandment or by arayot.
Derash limiting yibbum where an independent prohibition exists.
Yibbum is not performed when the yevamah is independently forbidden to the yavam.