There's a Drow Rogue (trickster) in our group and I'm trying to build his character on the Character Builder.

He's taken these two feats:

Xen'drik Weapon Training - which gives prof and +2 damage with Drow Long Knife and Xen'drik Boomerang. Important note is that the Drow Long Knife is a Heavy Blade.

Next he took the feat Verstile Duelest - which gives prof with Military Heavy Blades (which he doesn't really need) and it also allows you to use a Heavy Blade on rogue powers that require a Light Blade.

By my calculations, he should now be able to use his Drow Long Knife (a heavy blade with which he is prof) with his Rogue powers. But the Character Builder isn't calculating it for me.

Furthermore, if I swap the main hand weapon from a Drow Long Knife to a Longsword (another Heavy Blade) it does calculate it.

The only thing I can think of is that the Drow Long Knife is a superior weapon, and doesn't qualify.

I need two bits of help:
1 - help explain why it's not working (unless I've figured it out with the superior thing)

2 - help me get around it in the builder, as the DM has already approved him using it. I'd like to have the character sheet represent this correctly.

As always, thanks.