exporting null value to AD Accountexpires

Dear All,

I am trying to delete existing accountexpires value. using following c# script but no luck

long iFileTime = 9223372036854775807;
                    if (mventry["employeeEndDate"].ToString() != null)
                    {
                        DateTime dtFileTime = DateTime.ParseExact(mventry["employeeEndDate"].Value, "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.000'", provider);

                        csentry["accountExpires"].IntegerValue = dtFileTime.ToFileTimeUtc();
                    }
                    else
                    {
                        csentry["accountExpires"].IntegerValue = iFileTime;
                    }

Need Your Help!

Thanks,

Shashidhar