The remote endpoint could not be called, or the response it returned was in

I'm writing a skill as a Lambda Python function and I'm missing *something* in my response that's causing the service to think it's invalid. My lambda function is being called, and being executed fine, here is the output it's returning { "version": "1.0", "response": { "outputSpeech": { "text": "The next train is at 1649", "type": "PlainText" }, "shouldEndSession": false, "card": { "content": "1649", "type": "Simple", "title": "Train times" } }, "sessionAttributes": {} } I can't find anything wrong with it. Both the simulator and my Echo choke on it. Am I missing something obvious?

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