Uncaught TypeError: mutation.target.className.indexOf is not a function

I’m getting this error in the console of latest Chrome/Win7:

q2w3-fixed-widget.min.js?ver=5.0.4:37 Uncaught TypeError: mutation.target.className.indexOf is not a function
    at q2w3-fixed-widget.min.js?ver=5.0.4:37
    at Array.forEach (<anonymous>)
    at MutationObserver.<anonymous> (q2w3-fixed-widget.min.js?ver=5.0.4:36)

The part in the unminified code:

		var htmlObserver = new MutationObserver(function(mutations) {  
			mutations.forEach( function(mutation) {
				if ( q2w3_exclude_mutations_array(q2w3_sidebar_options).indexOf(mutation.target.id) == -1 && mutation.target.className.indexOf('q2w3-fixed-widget-container') == -1 ) {
					q2w3Refresh = true;
					//console.log('Mutation detected!');
				}
			});
	    });