I'm not sure what causes this error message, its seems random and hard to reproduce but here it is:

[00:15:31] ...terface\AddOns\ag_UnitFrames\modules\auras\auras.lua:718 attempt to index local 'c' (a nil value) [C]: ? ...terface\AddOns\ag_UnitFrames\modules\auras\auras.lua:718 in function `ScanAuras' ...terface\AddOns\ag_UnitFrames\modules\auras\auras.lua:546 in function `UpdateAuras' ...terface\AddOns\ag_UnitFrames\modules\auras\auras.lua:196 in function `?' Interface\AddOns\ag_UnitFrames\ag_UnitFrames.lua:461 in function <Interface\AddOns\ag_UnitFrames\ag_UnitFrames.lua:450