Element 'Envelope' was not found. Line 1, position 2.

Hi
I am acall a .Net 2.0 web service from my app - but I seem to get an error of "Element 'Envelope' was not found. Line 1, position 2." when I can one method.  I am passing the web web service a string which then return a datatable as a response,.  All my other methods work on the web service - the coee is identicakl.  I am dont understand why thsi method fails

Line 97:         [System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://tempuri.org/GetJob", RequestNamespace="http://tempuri.org/", ResponseNamespace="http://tempuri.org/", Use=System.Web.Services.Description.SoapBindingUse.Literal, ParameterStyle=System.Web.Services.Protocols.SoapParameterStyle.Wrapped)]
Line 98:         public System.Data.DataTable GetJob(string JobNumber) {
Line 99: object[] results = this.Invoke("GetJob", new object[] {Line 100:                        JobNumber});
Line 101:            return ((System.Data.DataTable)(results[0]));