If you attempt to click on a guild bank with zero bank tabs, you get a script error.
If you add a new guild bank tab, you get the following error:

Date: 2015-03-17 21:02:11
ID: 1
Error occured in: Global
Count: 1
Message: ..\AddOns\BagBrother\Events.lua line 110:
attempt to concatenate a nil value
Debug:
(tail call): ?

BagBrother\Events.lua:110: ?()
BagBrother\Startup.lua:20:
BagBrother\Startup.lua:20
Locals:
(_temporary) = "GUILDBANKBAGSLOTS_CHANGED"
(_temporary) = "GUILDBANKBAGSLOTS_CHANGED"
(*temporary) = defined =C:-1