getDevices with production state filtering returns no devices

I am calling the JSON API using Java on Zenoss 4.2.3, specifically the getDevices call which works great until I add a parameter to filter the devices by their production state. If I do this I get no devices at all. Have tried passing in both the numeric (eg 1000) and string value (eg Production) to no avail. I am using "productionState" as the key in the params object eg. params.put("productionState", "1000");

Using anything other than "productionState" as the key returns an error so it seems to be the correct parameter name, but I'm always getting back 0 devices.

Thanks.