Combat Log Filter trouble

[Angr]

I'm trying to use the new COMBAT_LOG_EVENT filters but so far they doesn't seem to filter much. In my OnEnable I have

self:RegisterEvent("COMBAT_LOG_EVENT")
CombatLogAddFilter("SPELL_CAST_SUCCESS", UnitGUID("player"), nil)

which I handle with

function addon:COMBAT_LOG_EVENT(timestamp, event, srcGUID, srcName, srcFlags, dstGUID, dstName, dstFlags, ...)
    if event == "SPELL_CAST_SUCCESS" then
        --how come other player's casts show up here?
    end
end

Am I using the filters wrong?