How to handle Double.toString(overlayModel.getSalary()) when "OverlayModel.getSalary()" returns undefined value?

Hi:
Before value hasn't been set into to OverlayModel,if I did:
"Double.toString(overlayModel.getSalary())", a compilation error:""
Something other than a double was returned from JSNI method.....': JS
value of type undefined, expected double" will be thrown out.The
question is why gwt don't return default value(zero ) directly but
"Something" or "JS value of type undefined".How can I hand this
undefined type?
use if(..){} to check?If so,code will be verbose.

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