confirmation alert c#

i would like to check confirmaion from following c# but javascript error occured with "expected ;"

string strconfirm = "<script>if(!window.confirm('Are you sure?')){window.location.href='Pre_.aspx?custId='"+custId+"';}</script>";
CSM.RegisterClientScriptBlock(

this.GetType(), "Confirm", strconfirm, false)


anyone help