Pass Object to Web Service

Hi
I'm trying to pass an object to a web service. The object is xml
serialized. However, I'm getting an System.InvalidOperationException. I
have never used web service before, and I don't know if I am forgetting
something.

public string GetSlir(object Slir) {
object[] results = this.Invoke("GetSlir", new object[] {
Slir});
return ((string)(results[0]));
}


[WebMethod]
public string GetSlir (object Slir)
{
return "inside";
}