[Callback function in middle of timeline?]

I can't seem to get mine to work. Here's my code:

 

(Note: I have other buttons not shown in code that play the timeline after every scene.)

var tl = new TimelineMax({paused:true});
	
tl.add("scene01")
        .from(".bg1", .5, {left:"+=10%", autoAlpha:0}, "scene01")
        .add(insertBG("01.png",".testcard"), "scene01")
     .add("scene02").addPause()
       .to(".bg1", .5, {autoAlpha:0}, "scene02")
       .from(".bg2", .5, {left:"+=10%", autoAlpha:0}, "scene02");

function insertBG(bgName, cardName) {
     $(cardName).css('background-image', 'url(images/'+ bgName +')');	
}

It throws the following error at the .add(insertBG()) call :

 

"Cannot add undefined into the timeline; it is not a tween, timeline, function, or string."

 

The insertBG()  function works fine when called directly outside the timeline... What am I doing wrong? Thanks!