Why do I get null reference error in IE but not in Firefox?

due to layout reasons I needed to write a routine that checks if at least one checkbox is checked on a specific form (in typo3). The routine also handles diff. languages so it can be translated...

But somehow IE (I tried with IE11) throws a null reference error when this line is reached (but in Firefox it works perfectly):

if (elems[i].children[0].control['checked'] === true

here is the whole routine:

     if(document.getElementById("pollFR") != null &&  document.getElementById("pollFR") != '' ){
        document.getElementById("pollFR").addEventListener("submit", submitCheckboxHandler);
        language = "FR"
    }

//some more identical if-statements, because there is a form for every //language e.g. pollEN, etc.

    function submitCheckboxHandler(ev) {
        var elems = document.getElementsByClassName('form-check');
        var elemCounter = 0;
        var i;
        for (i = 0; i < elems.length; i++) {
            if (elems[i].children[0].control['checked'] === true)
                elemCounter++;
        };
        if (elemCounter === 0) {
            switch(language){
                case "EN":
                    alert("Please choose at least one parameter!");
                    break;
                case "FR":
                    alert("...french...");
                default:
                    alert("...german...");
            }
            ev.preventDefault();
        }
    }

Does anyone know why this happens ?

Thanks !