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/trap.c

@@ -2162,7 +2162,7 @@ trapeffect_poly_trap(
             deltrap(trap);      /* delete trap before polymorph */
             newsym(u.ux, u.uy); /* get rid of trap symbol */
             You_feel("a change coming over you.");
-            polyself(0);
+            polyself(POLY_NOFLAGS);
         }
     } else {
         boolean in_sight = canseemon(mtmp) || (mtmp == u.usteed);