Calling ProxyClient methods with NULL-Parameters (for which a valid ParamConverter exists) causes an NPE at "object.getClass()":

public String toString(Object object, Type type, Annotation[] annotations)
{
   return providerFactory.toString(object, object.getClass(), type, annotations);
} 

Can't we simply use the known type (passed as parameter "type") if "object == null"?

Then the registered ParamConverters get a chance to handle the null value and convert it to some String that can be passed.

In my experiments (using the debugger) "type == object.getClass()" was always true, except when object is null (type is still correct).

Server-side this apparently works fine, already.