error: element is null

[GoogleCodeExporter]

Since I added the datepicker widget to my page I get the error 'element is 
null' on line 3847 of the prototype library.
That is this line: if (element._prototypeEventID) return 
element._prototypeEventID[0];

Any idea why this happens? This is my code:
var dpck_startdatum = new DatePicker({relative:"startdatum",language:"nl"});
var dpck_einddatum = new DatePicker({relative:"einddatum",language:"nl"});

Original issue reported on code.google.com by [email protected] on 21 Aug 2010 at 12:46