Can't remove a User Group

JavaScript - http://www.luxburg.de/manager/index.php?a=47

Uncaught exception: TypeError: 'this.indexOf' is not a function
Error thrown at line 7, column 12752 in <anonymous function: remove>(b):
    var a=this.indexOf(b);
called from line 16, column 16 in <anonymous function: handler>(itm, e):
    this.remove(itm,e);
called via Function.prototype.apply() from line 7, column 7160 in <anonymous function: fire>():
    var o=Array.prototype.slice.call(arguments,0);
called via Function.prototype.apply() from line 7, column 650 in <anonymous function: fireEvent>():
    p.push(l)
called from line 11, column 64500 in <anonymous function: onClick>(a):
    if(!this.disabled&&this.fireEvent("click",this,a)!==false&&(this.parentMenu&&this.parentMenu.fireEvent("itemclick",this,a)!==false))
called from line 11, column 45232 in <anonymous function: onClick>(b):
    a.onClick(b);
called via Function.prototype.call() from unknown location in K(O):
    /* no source available */