Web Services and Powershell error: Method can not be reflected

I'm trying to use the VSM 9.1 Web Service API with Powershell, however when I run the New-WebServiceProxy cmdlet it throws an error, the inner exception being "Method InfraAPI.KnowledgeUpdate can not be reflected.", and as such I'm unable to proceed any further with development.

Here's the Powershell output:

PS C:\PS> new-webserviceproxy -uri http://vsmhost/dev/ServiceManager.svc?wsdl

New-WebServiceProxy : Exception has been thrown by the target of an invocation.

At line:1 char:20

+ new-webserviceproxy <<<<  -uri http://vsmhost/dev/ServiceManager.svc?wsdl

    + CategoryInfo          : NotSpecified: (:) [New-WebServiceProxy], TargetInvocationException

    + FullyQualifiedErrorId : System.Reflection.TargetInvocationException,Microsoft.PowerShell.Commands.NewWebServiceProxy

PS C:\PS> $error[0].exception.innerexception

Method InfraAPI.KnowledgeUpdate can not be reflected.

Any help with this would be greatly appreciated.