jToker / Devise Token Auth demo has a 500 internal error.

I was testing out the demo registering and the post was getting a 500 error.

main.js:16 POST https://devise-token-auth-demo.herokuapp.com/auth 500 (Internal Server Error)
send @ main.js:16
ajax @ main.js:16
F.emailSignUp @ main.js:34
handleRegistrationClick @ main.js:3
h @ main.js:25
executeDispatch @ main.js:32
g @ main.js:25
i @ main.js:25
l @ main.js:25
c @ main.js:32
processEventQueue @ main.js:25
c @ main.js:30
handleTopLevel @ main.js:30
e @ main.js:30
perform @ main.js:32
batchedUpdates @ main.js:28
f @ main.js:31
dispatchEvent @ main.js:30

VM534:1 Uncaught SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at Function.ea.parseJSON (main.js:16)
    at F.rejectPromise (main.js:34)
    at F.error (main.js:34)
    at k (main.js:13)
    at Object.fireWith [as rejectWith] (main.js:13)
    at c (main.js:16)
    at XMLHttpRequest.b (main.js:16)

Start by reproducing registration from the linked jToker demo and inspecting the server-side cause of the POST /auth 500 response. Check why the error response is not valid JSON; done means registration succeeds and the demo receives a valid response.