With... Object subclass: #BittrexResponse instanceVariableNames: 'success message result' classVariableNames: '' package: 'Bittrex' Object subclass: #BittrexMarketSummary instanceVariableNames: 'MarketName High Low Volume Last BaseVolume TimeStamp Bid Ask OpenBuyOrders OpenSellOrders PrevDay Created DisplayMarketName' classVariableNames: '' package: 'Bittrex' this code works great when the response holds good data... ZnClient new url: 'https://bittrex.com/api/v1.1/public/getmarketSummary?market=BTC-LTC'; enforceHttpSuccess: true; accept: ZnMimeType applicationJson; contentReader: [ :entity | |reader| reader := (NeoJSONReader on: entity readStream). reader for: BittrexResponse do: [:m| m mapInstVar: #success. m mapInstVar: #message. (m mapInstVar: #result) valueSchema: #ResultArray]. reader for: #ResultArray customDo: [ :mapping | mapping listOfElementSchema: BittrexMarketSummary ]. reader mapInstVarsFor: BittrexMarketSummary. reader nextAs: BittrexResponse ]; get. i.e. a raw response looking like this.... (ZnClient new url: 'https://bittrex.com/api/v1.1/public/getmarketsummary?market=BTC-LTC <https://bittrex.com/api/v1.1/public/getticker?market=BTC-LTC>'; get) inspect. ==> "'{""success"":true,""message"":"",""result"":[{""MarketName"":""BTC-LTC"",""High"":0.01982450,""Low"":0.01285257,""Volume"":1436429.81313360,""Last"":0.01842000,""BaseVolume"":24841.17217724,""TimeStamp"":""2017-12-13T05:56:25.937"",""Bid"":0.01840001,""Ask"":0.01842000,""OpenBuyOrders"":10140,""OpenSellOrders"":6306,""PrevDay"":0.01439800,""Created"":""2014-02-13T00:00:00""}]}'" But for bad response looking like this... (ZnClient new url: 'https://bittrex.com/api/v1.1/public/getmarketsummary?market=INVALID'; get) inspect. ==> {"success":false,"message":"INVALID_MARKET","result":null} the JSON handling code fails deep in the call stack with an error "NeoJSONParseError: [ expected" which is not so friendly for users of the Bittrex library. What are the different/recommended approaches with Zinc for catching JSON errors such that I can pass "message" as a higher level Error up the stack to the Bittrex user. cheers -ben