Grab computed background-size on Internet Explorer with jQuery

Question

I have this style:

#bkg-evs-1 {
    background-color: white!important;
    background-image: url("../img/rip-van-winkle-copy8-adj.jpg"), url("../img/Background-events2.jpg");
    background-repeat: no-repeat, no-repeat;
    background-size: 29% 100vh, 70%;
    background-position: top right, 0% 0%;
    background-attachment: fixed, scroll;
    display: none;
    min-height: 2500px;
    width: 100%;
    position: absolute;
    top: 0;
    left: 0;
}

Whenever I try to get the background-size on internet Explorer 11 using:

jQuery(document).ready(function(event) {
    var bs = jQuery("#bkg-evs-1").css("background-size");
    console.log(bs);
});

I always get 0px 0px, 0px on the console.

Now,

• with other browsers I always get the right computed background-size (which depends of the window size), no problems.
• I have tried using pure javaScript as well without results.
• I have also noticed that with internet explorer 11, if I use this CSS background-size: 300px 300px, 300px 300px; there is no problems, the computed size will result on the console: 300px 300px, 300px 300px.
• And, finally, in internet explorer 11, if I write directly in the console: jQuery("#bkg-evs-1").css("background-size") the console will give me the right values. So, the problem only happens when the page loads.

Am I doing something wrong?