Error programaticaly rendering a report using Reporting Services Web Services

I'm using Reporting Services 2005 SP1 and wrote some code to render reports server-side.
My sample report has few parameters that must be passed so I pass these parameters with code.

<My code>
ReportServiceExecution.ReportExecutionService rs = new ReportServiceExecution.ReportExecutionService();
rs.Credentials = System.Net.CredentialCache.DefaultCredentials;
string extension, mimeType, encoding;
ReportServiceExecution.Warning[] warnings;
string[] streamID;

ParameterValue[] parameters = new ParameterValue[2];

parameters[0] = new ParameterValue();
parameters[0].Name = "pID";
parameters[0].Value = "6548747";

parameters[1] = new ParameterValue();

parameters[1].Name = "pClass";

parameters[1].Value = "8";

ExecutionHeader header = new ExecutionHeader();
ExecutionInfo executionInfo = rs.LoadReport(reportPayslip.Name, null);
executionInfo = rs.SetExecutionParameters(parameters, "en-us");
rs.ExecutionHeaderValue = header;
rs.ExecutionHeaderValue.ExecutionID = executionInfo.ExecutionID;

byte[] report = rs.Render("PDF", "<DeviceInfo><Toolbar>False</Toolbar></DeviceInfo>", out extension, out mimeType, out encoding, out warnings, out streamID);
</My code>

Actually, I have several more parameters but I've checked the collection and all the parameters I want are there and have values. Their names match those found in executionInfo.Parameters. But when I run my code I get the following error on Render():

<Error>
"This report requires a default or user-defined value for the report parameter 'pClass'. To run or subscribe to this report, you must provide a parameter value. > This report requires a default or user-defined value for the report parameter 'pClass'. To run or subscribe to this report, you must provide a parameter value. > This report requires a default or user-defined value for the report parameter 'pClass'. To run or subscribe to this report, you must provide a parameter value."
</Error>

I've double-checked and my parameter array does have the problematic parameter, I passed it a value but when I check executionInfo.Parameters I see that it's the only parameter that hasn't been give a default value after calling SetExecutionParameters(). So my parameters do seem to be passed but one seems to refuse getting a value. If I comment out the SetExecutionParameters() line I get, as expected a similar error but on another parameter (first one in the executionInfo.Parameters collection). So I'm left to beleive that the SetExecutionParameters() has some kind of bug as it works for all but one parameter. I've checked casing, spelling, tried passing phony parameters and from what I've seen I should be getting an error when I pass the parameters if I got something wrong.

Any help would be much appreciated.

I've done some (desperate) tests and changed the following line:
executionInfo = rs.SetExecutionParameters(parameters, "en-us");
to
executionInfo = rs.SetExecutionParameters(parameters, "fr-ch");
I'm currently based in French Switzerland but our reports are written in english, besides, the documentation about this parameter is basically "A .Net language" which is not very helpful. So now it works but I'm afraid if I have to access a server that is of another culture it will fail.