Does not appear to work with gRPC methods (or `AsyncUnaryCall` return types)

[Smurf-IV]

gRPC functions create the following style of method:
public virtual grpc::AsyncUnaryCall<global::Proto.GetLoadingConditionsReply> GetLoadingConditionsAsync(global::Proto.GetLoadingConditionsRequest request, grpc::CallOptions options)

This is added as a standard wrapper:
myRepositoryCache = FluentMemoryCache.WithSource(aClient);
aClient being the instance of the gRPC client class.

Then because the loadingCondition async API has options, I have to create a set CallOptions
var callOptions = new CallOptions().WithHeaders(metadata);

And then attempt to code the following:

Which you can see is causing problems