UnitInVehicle() not working inside an event handler

[FranekW]

I have made a small addon which is supposed to hide my player and target frames when I enter a vehicle and restore their previous states on exit. The addon is working and all events are registered and fired as expected. The only problem I have is one of the function UnitInVehicle() is somehow not recognised in the event handler UNIT_EXITED_VEHICLE. Probably I am missing something obvious.

What happens is that the function either causes error when used directly because it is nil or is always recognised as false when used inside "if". I don't understand it because, when I print UnitInVehicle("player") in console, it correctly returns boolean value.

HiddenFrames = LibStub("AceAddon-3.0"):NewAddon("HiddenFrames", "AceEvent-3.0")
 
function HiddenFrames:OnInitialize()
    -- Called when the addon is loaded
    self.PlayerFrameAlphaHidden = 0
    self.TargetFrameAlphaHidden = 0
end
 
function HiddenFrames:OnEnable()
    self.PlayerFrameAlphaOn = PlayerFrame:GetAlpha()
    self.TargetFrameAlphaOn = TargetFrame:GetAlpha()
    --
    self:RegisterEvent("UNIT_ENTERED_VEHICLE")
    self:RegisterEvent("UNIT_EXITED_VEHICLE")
end
 
-- function HiddenFrames:OnDisable()
--     -- Called when the addon is disabled
-- end
 
function HiddenFrames:UNIT_ENTERED_VEHICLE(eventName, unitId)
    if unitId == "player" then
        PlayerFrame:SetAlpha(self.PlayerFrameAlphaHidden)
        TargetFrame:SetAlpha(self.TargetFrameAlphaHidden)
    end
end
 
function HiddenFrames:UNIT_EXITED_VEHICLE()
    if UnitInVehicle("player") then
        print("Inside if")
        PlayerFrame:SetAlpha(self.PlayerFrameAlphaOn)
        TargetFrame:SetAlpha(self.TargetFrameAlphaOn)
    end
end

EDIT.
I have changed the handler to check directly the second parameter sent to a function and it is working but I still don't get why I have the error when trying to check if a player is inside a vehicle!

function HiddenFrames:UNIT_EXITED_VEHICLE(eventName, unitId)
    if unitId == "player" then
        PlayerFrame:SetAlpha(self.PlayerFrameAlphaOn)
        TargetFrame:SetAlpha(self.TargetFrameAlphaOn)
    end
end