Error exporting CSR with iControl

I'm trying to automate the process of creating vip/pool/ssl sets for our QA-Test systems, which are built dynamically using VMs and iControl. I have all the entity creation stuff working, but getting stuck on SSL Cert/Key creation. I successfully create a key, I successfully generate a CSR. I need to sign the CSR with an external CA (Self-hosted) so I need to get the file. Right now I am trying to do this using the certificate_request_export_to_file() feature of iControl, invoked via the .NET library. So I do

result = this.ManagementKeyCertificate.Begincertificate_request_export_to_file(ManagementKeyCertificateManagementModeType.MANAGEMENT_MODE_DEFAULT, new string[] { req.id }, new string[] { @"/path/where/I/want" }, true, null, null);

this.ManagementKeyCertificate.Endcertificate_request_export_to_file(result);

And I get a nasty error, almost like the CGI portal for icontrol bombed on me: "Client found response content type of 'text/html; charset=iso-8859-1', but expected 'text/xml'.\r\nThe request failed with the error message:\r\n--\r\n \r\n--." I am catching the traffic coming over the wire, but the SOAP response is blank. just text/xml, headers no body. HTTP Code: 500. Everything else works, but I continually hit this. What am I missing?