What is this internal ASP.net error refering to ?

Hi;

 I am trying to debug my web application in IE and I get this error message "jscript runtime error: 'this._form is null or not an object'?

The script debugger stops on the following line in function Sys$WebForms$PageRequestManager$_initializeInternal(scriptManagerID, formElement) { this.(notice the period)_scriptManagerID = scriptManagerID;

this._form._initialAction = this._form.action; line 681

Thanks for any insights !