The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
66. Masculine and Feminine parisyllabic nouns in
-is form the Nominative singular by
adding s to the stem.
Four stems in bri- and tri- do not add s to form the nominative, but drop i and insert e before r.
These are imber, linter, úter,