print HTMLCollection works, but htmlCollection.length = 0

I have a strange problem.

console.log(htmlCollection) outputs all items as expected. But htmlCollection.length returns 0. Converting it to an array or accessing items per index fails also.

My function:

function findElement(htmlCollection, content){
  console.log(htmlCollection);
  console.log("Length: " +htmlCollection.length);
}

Output:

HTMLCollection []
  0: ytmusic-menu-navigation-item-renderer.style-scope.ytmusic-menu-popup-renderer.iron-selected
  1: ytmusic-menu-service-item-renderer.style-scope.ytmusic-menu-popup-renderer
  2: ytmusic-menu-service-item-renderer.style-scope.ytmusic-menu-popup-renderer
  3: ytmusic-toggle-menu-service-item-renderer.style-scope.ytmusic-menu-popup-renderer
  4: ytmusic-menu-navigation-item-renderer.style-scope.ytmusic-menu-popup-renderer
  5: ytmusic-menu-service-item-renderer.style-scope.ytmusic-menu-popup-renderer
  6: ytmusic-menu-navigation-item-renderer.style-scope.ytmusic-menu-popup-renderer
  7: ytmusic-menu-navigation-item-renderer.style-scope.ytmusic-menu-popup-renderer
  8: ytmusic-menu-service-item-renderer.style-scope.ytmusic-menu-popup-renderer
  length: 9
  __proto__: HTMLCollection

Length: 0

Do you have any ideas what the problem could be?