Generate SSRS report in PDF from WebService throws error

Question

Im trying to generate SSRS report and export it to PDF using webservice(asmx) but getting the following error when try to set datasource credentials. Previously I have done it in WebApi and it works fine.

Value does not fall within the expected range. at Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.GetSecureMethods() at Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.IsSecureMethod(String methodname) at Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.SetConnectionSSLForMethod(String methodname) at Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.ProxyMethodInvocation.Execute[TReturn](RSExecutionConnection connection, ProxyMethod1 sql16Method, ProxyMethod1 katmaiMethod, ProxyMethod`1 yukonMethod) at Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.LoadReport(String Report, String HistoryID) at Microsoft.Reporting.WebForms.SoapReportExecutionService.LoadReport(String report, String historyId) at Microsoft.Reporting.WebForms.ServerReport.EnsureExecutionSession() at Microsoft.Reporting.WebForms.ServerReport.GetDataSources(Boolean& allCredentialsSet) at Microsoft.Reporting.WebForms.ServerReport.GetDataSources() at OtoHRImportServices.Repository.PayslipRepo.CreatePayslip(OtoHREntities db, String conn_string, String report_root_path, PayslipVM model, String password)

List<DataSourceCredentials> dsCredentials = new List<DataSourceCredentials>();
dsCredentials.Add(cred);
ReportViewer1.ServerReport.SetDataSourceCredentials(dsCredentials); <-- throw error