CalendarExtender and javascript error

Hi,

Did anyone run into the following javascript error while using the CalendarExtender?

Uncaught Sys.ArgumentException: Sys.ArgumentException: Value must not be null for Controls and Behaviors. Parameter name: element

The error appears in the following javascript function (chrome browser):

$create = $type.create = function Component$create(type, properties, events, references, element) {
    /// <summary locid="M:J#Sys.Component.create">Instantiates a component of the specified type, attaches it to the specified element if it's  a Control or Behavior, sets the properties as described by the specified JSON object,  then calls initialize.</summary>
    /// <param name="type" type="Type">The type of the component to create.</param>
    /// <param name="properties" optional="true" mayBeNull="true">A JSON object that describes the properties and their values.</param>
    /// <param name="events" optional="true" mayBeNull="true">A JSON object that describes the events and their handlers.</param>
    /// <param name="references" optional="true" mayBeNull="true">A JSON object that describes the properties that are references to other components.  The contents of this object consists of name/id pairs.  If in a two-pass creation, the setting of these properties will be delayed until the second pass.</param>
    /// <param name="element" domElement="true" optional="true" mayBeNull="true">The DOM element the component must be attached to.</param>
    /// <returns type="Object">The component instance.</returns>
    var e = Function._validateParams(arguments, [
        {name: "type", type: Type},
        {name: "properties", mayBeNull: true, optional: true},
        {name: "events", mayBeNull: true, optional: true},
        {name: "references", mayBeNull: true, optional: true},
        {name: "element", mayBeNull: true, domElement: true, optional: true}
    ]);
    if (e) throw e;
    if (type.inheritsFrom(Sys.UI.Behavior) || type.inheritsFrom(Sys.UI.Control)) {
        if (!element) throw Error.argument('element', Sys.Res.createNoDom);
    }
    else if (element) throw Error.argument('element', Sys.Res.createComponentOnDom);
    var component = (element ? new type(element): new type());
    callIf(component, "beginUpdate");
    if (properties) {
        Sys.Component._setProperties(component, properties);
    }
    if (events) {
        for (var name in events) {
            if (!(component["add_" + name] instanceof Function)) throw new Error.invalidOperation(String.format(Sys.Res.undefinedEvent, name));
            if (!(events[name] instanceof Function)) throw new Error.invalidOperation(Sys.Res.eventHandlerNotFunction);
            component["add_" + name](events[name]);
        }
    }
    Sys.Component._register(component, references);
    return component;
}

I am building a dynamic form and I add the extender and the textbox control from codebehind.

Unfortunatelly I sometimes get the above javascript error. I have no idea why in some cases it appears and others it doesn't.

I would really appreciate your help with that.

Thanks