Office UI Fabric JS DropDown not working in IE11

Question

I've got a SharePoint Add-In with two drop downs:

<div id="departments" class="ms-Dropdown ms-Grid-col ms-u-sm3" tabindex="0">
    <label class="ms-Label">Departments</label>
    <i class="ms-Dropdown-caretDown ms-Icon ms-Icon--ChevronDown"></i>
    <select class="ms-Dropdown-select" id="departmentSelect">
        <option value=""></option>
    </select>
</div>
<div id="supervisors" class="ms-Dropdown ms-Grid-col ms-u-sm3" tabindex="0">
    <label class="ms-Label">Supervisors</label>
    <i class="ms-Dropdown-caretDown ms-Icon ms-Icon--ChevronDown"></i>
    <select class="ms-Dropdown-select" id="supervisorSelect">
        <option value=""></option>
    </select>
</div>

Which are then initialized by:

var DropdownHTMLElements = document.querySelectorAll('.ms-Dropdown');
for (var i = 0; i < DropdownHTMLElements.length; ++i) {
    var Dropdown = new fabric['Dropdown'](DropdownHTMLElements[i]);
}

And all other browsers work, but IE11 never loads and throws that error:

"Unable to get property 'querySelectorAll' of undefined or null reference"
File: fabric.min.js, Line: 7, Column: 17876