WoWInterface - Help with PetFrame 'pet experience bar'

I'm trying to add a pet experience bar to my PetFrame xml, however so far I can only get it to display after visiting the Stable Master. I'm not sure what i'm doing wrong but perhaps someone can help me out here:

LUA:

Code:

function PetFrame_OnLoad()

      this:RegisterEvent("UNIT_PET_EXPERIENCE");

      PetFrameExpBar_Update();

end



function PetFrame_Update()

      PetFrameExpBar_Update();

end



function PetFrame_OnEvent(event)

      UnitFrame_OnEvent(event);

              if ( event == "UNIT_PET_EXPERIENCE" ) then

                PetFrameExpBar_Update();

        end

end



function PetFrameExpBar_Update()

        local currXP, nextXP = GetPetExperience();

        PetFrameExpBar:SetMinMaxValues(min(0, currXP), nextXP);

        PetFrameExpBar:SetValue(currXP);

end

XML:

Code:

<StatusBar name="PetFrameExpBar" inherits="TextStatusBar">

    <Size>

        <AbsDimension x="122" y="3"/>

    </Size>

        <Anchors>

                <Anchor point="TOPLEFT">

                <Offset>

                        <AbsDimension x="-5" y="-18"/>

                </Offset>

            </Anchor>

        </Anchors>

        <Scripts>

                <OnLoad>

                        PetFrameExpBar_Update();

                        TextStatusBar_Initialize();

                        this:Show();

                </OnLoad>

                <OnEvent>

                        PetFrameExpBar_Update();

                </OnEvent>

        </Scripts>

        <BarTexture file="Interface\TargetingFrame\UI-StatusBar"/>

                <BarColor r="1.0" g=".32" b="1.0"/>

</StatusBar>