Thanks to blizzard, you can no longer use custom weapons's unless there item "ID" is already in the game/ DBC's. You would get an error saying "you need to equip a weapon".

Here's a guide on how to get over it and use custom item's;

1. Create the custom weapon.

2. Go to this link for a list of unused item id's that are already in the dbc's and use an item id listed in it.

http://pastebin.ca/1799396

3. Execute the custom weapon into your database and it should work correctly.

Credits to:

deep6ixed for the unused weapon id list