Passing back a Map with a key fo type Date in RPC?

I am making a RPC call and included in the response is a HashMap that has Date objects as keys. All the data is being stripped out and in the smartgwt.log I see these messages:

<b>Ignoring unconvertible Map key of type: java.util.Date</b>

Is there any reason I am not allowed to set over a Map with the keys being a Date object?