Fetch to GraphQL server from Redux client returning before request finishes?

I'm trying to make a POST to a GraphQL server in a React Redux app.

retu (dispatch) => fetch(API_URL, {
   method: 'POST',
   body: JSON.stringify(query)
})
.then(response => response.json())
.then(json => dispatch({
  type: CREATE_USER,
  payload: json
}))

When I place a breakpoint in the response.json() part of the code and the browser pauses there, the network panel shows a 200 OK but no response and was me that my request is not finished yet.

The response object looks like this:

Response {
  body: null,
  bodyUsed: false,
  headers: Headers,
  ok: false,
  status: 0,
  statusText: "",
  type: "opaque",
  url: ""
}

The code never reaches the second then and the console shows an error SyntaxError: Unexpected end of input at SyntaxError.

When I let the rest of the code play out and check the network panel, my POST has completed successfully and retued the data I need.

I was wondering what's going on here? Why does the then function fire when the request is not yet done and the response body is null? How can I tell my code to wait until the request completes?