'IsReference' property specified was not found in WCF webservice

I am getting a lot of these exceptions:

The service '/Services/LookupListService.svc' cannot be activated due to an exception during compilation.
The exception message is: 'IsReference' property specified was not found..

at System.ServiceModel.AsyncResult.End[TAsyncResult](IAsyncResult result)
at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result)
at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(HttpApplication context, Boolean flowContext)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)



Has anyone else had something similar to this? The webapplication is running 3.5 on IIS7.
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