Invalid Lambda Response says Text can't be empty or null. How to fix?

I've programmed a lambda function to return an ElicitIntent with a "contentType" of "application/vnd.amazonaws.card.generic". When Lex receives the response it throws this error:

An error has occurred: Invalid Lambda Response: Received invalid response from Lambda: Can not construct instance of Button$Builder, problem: Text can't be empty or null

I don't understand "Text", with the capital T. There is no "Text" in my return response. I have "text" as part of my buttons but they are not empty.
I'm confused. Any help appreciated.