Cannot cast any spell in fight vs Cail the Silence. Possibly a bug?

[adaxe]

When fighting vs Cail the Silence(the dragon in Searing Fall cave), my wizard played the animation of casting, but failed to actually cast any spells. I am pretty sure that no one is hitting him(except the fear aura), so it is unlikely caused by interruption. Could it be a bug?

[Omnicron]

It is a bug 

 

http://forums.obsidian.net/topic/79498-kakaloths-minor-blights-bugs/