Zipcode validation

function Patientzipcode(sender, args)

 {

var zipcode = document.getElementById('<%= txtZipCode.ClientID%>').value;

if (zipcode == "00000") 

{

document.getElementById('<%= CustmZipcode.ClientID%>').innerHTML = "Please enter valid zipcode";

return args.IsValid = false;

}

else

{

return args.IsValid = true;

}

}