This happened right when I logged on:

2x ...Master\Libs\AceGUI-3.0-41-Spell-EditBox\SpellLoader.lua:217: attempt to index field '?' (a nil value)
[string "@RotationMaster\Libs\AceGUI-3.0-41-Spell-EditBox\SpellLoader.lua"]:217: in function UpdateFromSpellBook' [string "@RotationMaster\main.lua"]:1079: in function ?'
[string "@datastore\libs\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:119: in function <...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:119>
[string "=[C]"]: ?
[string "@datastore\libs\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:29: in function <...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:25>
[string "@datastore\libs\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:64: in function `Fire'
[string "@datastore\libs\AceEvent-3.0\AceEvent-3.0-4.lua"]:120: in function <DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:119>

I found the issue, it will be fixed next version.

546d1c6

I manually added your fix and ended up with a 2nd error along with the original:

10x ...Master\Libs\AceGUI-3.0-41-Spell-EditBox\SpellLoader.lua:76: bad argument #1 to 'lower' (string expected, got nil)
[string "=[C]"]: in function lower' [string "@RotationMaster\Libs\AceGUI-3.0-41-Spell-EditBox\SpellLoader.lua"]:76: in function <...Master\Libs\AceGUI-3.0-Spell-EditBox\SpellLoader.lua:71> [string "@RotationMaster\Libs\AceGUI-3.0-41-Spell-EditBox\SpellLoader.lua"]:216: in function UpdateFromSpellBook'
[string "@RotationMaster\main.lua"]:1079: in function ?' [string "@DataStore\libs\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:119: in function <...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:119> [string "=[C]"]: ? [string "@DataStore\libs\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:29: in function <...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:25> [string "@DataStore\libs\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:64: in function Fire'
[string "@datastore\libs\AceEvent-3.0\AceEvent-3.0-4.lua"]:120: in function <DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:119>

What class are you playing?
On what edition of WoW?

I am playing a hunter-beast master and it's the latest classic TBC edition.

Just to be sure, this only happens with my hunter, all others it works just fine. I even tried repeating it with just the one addon and it only errored out with the hunter.

This should be fixed, test it with the 1.4.1 Beta

Ok will do.

It works just fine, excellent!