Blessed potion of polymorph asks user for monster to poly into

... unless there's some other form that would override the choice,
such as a worn dragon armor, lycanthropy, or vampirism.

The polymorph will be in effect for 10-24 turns.

src/mhitm.c

@@ -992,7 +992,7 @@ mon_poly(struct monst *magr, struct monst *mdef, int dmg)
             /* system shock might take place in polyself() */
             if (u.ulycn == NON_PM) {
                 You("are subjected to a freakish metamorphosis.");
-                polyself(0);
+                polyself(POLY_NOFLAGS);
             } else if (u.umonnum != u.ulycn) {
                 You_feel("an unnatural urge coming on.");
                 you_were();