Wolfman Howl (Spell #998) doesn't summon wolves

[Kafkaa]

Supposed to summon 2 rabid dire wolves. Target: Self -- Attack Type: Cold -- Summon rabid dire wolf(802), Summon rabid dire wolf(802)

You slice and dice short wolfman for 69 damage!
You slice and dice short wolfman for 29 damage!
You slice and dice short wolfman for 59 damage!
You slice and dice short wolfman for 69 damage!
You slice and dice short wolfman for 30 damage!
You slice and dice short wolfman for 59 damage!
The small wolfman raises its head and a howl echoes into the air!
The thin wolfman raises its head and a howl echoes into the air!
The short wolfman swipes at you with its claws!
The short wolfman swipes at you with its claws!
The short wolfman swipes at you with its claws!
The small wolfman lunges at you!
The small wolfman swipes at you with its claws!
The small wolfman swipes at you with its claws!
The thin wolfman swipes at you with its claws!
The thin wolfman swipes at you with its claws!
The thin wolfman swipes at you with its claws!
You critically slice and dice short wolfman for 285 damage!
5 silver drop to the ground.
36 copper drop to the ground.
The wolfman lets out a final anguished howl before collapsing.
You gain 7000 experience.
*Combat Off*
[HP=729]:
Blackwood Forest
You notice 16 gold crowns, 75 silver nobles, 146 copper farthings, severed arm,
severed leg, decapitated head here.
Also here: small wolfman, thin wolfman.
Obvious exits: north, southwest
The room is dimly lit

Should have been 4 wolves in there, since they both cast it.

[Kafkaa]

Just ran into a group of these guys that summoned wolves. Is it only a percentage chance thing?