Div not showing with JavaScript

I’m currently improving my website for the use with mobile devices. Therefore I’m trying to open a menu with a click onto a equiv button. The menu is a fixed div with scrollable content and should appear and disappear with a click onto the button. Either option is working if set through the display property in CSS, but I’m not able to toggle the visibility through JavaScript.

function ToggleDiv() {
  var v = document.getElementById('equiv-submenu');

  if (v.style.display == '' || v.style.display == 'none') {
    v.style.display = 'block';
  }
  else {
    v.style.display = 'none';
  }
}

<div id="equiv" onclick="ToggleDiv()">
  <p>&equiv;</p>
</div>

<div id="equiv-submenu">
  <div id="equiv-submenu-content">
    ...
  </div>
</div>