Cannot convert to same type in WCF

I am building a test-stub for a webservice, implementing the interface retrieved from the production webservice using svcutil. When calling a method on the stub i get the exception;

Object of type 'Sbsys.Services.HostService.DokumentBoks.DKALWSAfsendService.MaterialeType[]'cannot be converted to type'Sbsys.Services.HostService.DokumentBoks.DKALWSAfsendService.MaterialeType[]'.

where 'Sbsys.Services.HostService.DokumentBoks' is the namespace of the consuming client, and 'DKALWSAfsendService' is the namespace containing the generated proxy classes for the service.

Any ideas on whats going on?

Bonus info: When using the production service everything works as intended