GWT error "no instantiable subtypes" with interface in RPC method with 1 known concrete class?

I have added the following async remote service method (using 2.7.0.):

TransportTokenResponse<ProductDto> findActiveDeclarationsOfActiveMember(TransportTokenRequest<Void> request);

TransportTokenResponse, just like TransportTokenRequest are interfaces with both an abstract subclass and one concrete class.

The GWT console shows the following error:

   ERROR] 'com.ited.lang.transporttoken.TransportTokenResponse' has no instantiable subtypes 

I am aware that using interfaces in RPC methods is tricky and can result in bloat code, but in this case, I only have one concrete class, so why does GWT complain and can't use the concrete class (it's in the same package as the interface) ?

I noticed this is a new 2.7.0. warning, maybe it has something to do with this

The details of TransportTokenResponse,:

public abstract class TransportTokenResponseBase<T> implements TransportTokenResponse<T> {
 private static final long serialVersionUID = 1L;

 protected TransportTokenResponseBase() {
 }
}


public final class SimpleTransportTokenResponse<T> extends TransportTokenResponseBase<T> {
 private static final long serialVersionUID = 1L;
    SimpleTransportTokenResponse() {
 }
}

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