warning when compiling RPC method with long[] return type

Hello,

I have an RPC interface:

public interface Generator extends RemoteService {
long generate();
long[] generate(int count);
}

I get the warning during compiling:
[WARN] Line 14: Referencing class 'long_Array_Rank_1_FieldSerializer:
unable to resolve class, expect subsequent failures

What does this mean?
I use gwt-1.6.1, and there was no warning until I upgraded from 1.5.3.

Thanks for your help!
Regards:
Norbi

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