Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)

I have two fields avd_starttime and avd_stoptime. I am writing the below code to update the avd_stoptime field with datetime.now. But I am getting the error as "Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)."..PLease tell me where I am going wrong??

 internal static bool StopAht(ICrmContext context)
        {
            QueryExpression exp = new QueryExpression(avd_agenthandlingtime.EntityLogicalName);
            exp.NoLock = true;
            exp.ColumnSet.AddColumn(XrmAttributes.AgentHandlingTime.Id);
            exp.ColumnSet.AddColumn(XrmAttributes.AgentHandlingTime.StopTime);

            ConditionExpression CondOppId = new ConditionExpression();
            CondOppId.AttributeName = "avd_StopTime";
            CondOppId.Operator = ConditionOperator.Null;

            string timerId = XrmAttributes.AgentHandlingTime.Id;

            var coll = context.OrganizationService.RetrieveMultiple(exp);
            if (coll != null && coll.Entities != null && coll.Entities.Count > 0)
            {
                foreach (var ent in coll.Entities)
                {
                    avd_agenthandlingtime stopTimer = new avd_agenthandlingtime()
                    {
                        Id = new Guid(timerId),
                        avd_StopTime = DateTime.Now
                    };
                    context.AdminUpdate(stopTimer);

                }
            }
            return true;
        }