Promise reject not triggering

I'm relatively new to Promises in js and am having trouble understanding why the following block of code does not run my catch function when the server responds with a 401 unauthorized.

loginUser(email, password).then((token) => {
  console.log("in then")
  //ipcRenderer.send('login-success', token)
}).catch(err => {
  console.log("in catch")   //not running
})

The loginUser function:

function loginUser(email, password) {
  let body = { email: email, password: password  }

  return fetch('http://localhost:3000/api/v1/sessions', {
    method: 'POST',
    body: JSON.stringify(body),
    headers: { 'Content-Type': 'application/json' }
    }).then(response => {
       return response.json().then(json => {
         console.log(response.ok) // false
         return response.ok ? json : Promise.reject(json)
    }).catch(err => {
      console.error(err)
    })
  })
}

Any help at all would be appreciated. Cheers