HTML5 video error handling

I need to tell, whether video cannot be played ("x" sign is shown in browser).

This code does't works. "onerror" event will never be fired under Firefox

var v = document.getElementsByTagName("video")[0];
    if ( v != undefined )
        v.onerror = function(e) {
            if ( v.networkState == v.NETWORK_NO_SOURCE )
            {
                // handle error
            }
        }

What's wrong here ?