How to use ember-apollo-client?

I am trying to use graphql through apollo-client. On frontend there is Ember, on backend we have django and graphql server, implemented on this tutorial (https://www.howtographql.com/graphql-python/0-introduction/). The Ember application is on 4200 port, the graphql server is proxied on 8000/graphql (ember s --proxy http://localhost:8000)

I tried to follow this guide (https://www.howtographql.com/ember-apollo/1-getting-started/), but without graphcool, because we have backend.

In a controller signup.js I have

import Controller from '@ember/controller';
import { inject as service } from '@ember/service';
import Mutation from '../gql/mutation';

I have injected apollo this way

export default Controller.extend({
    apollo: Ember.inject.service(),

And the mutation is this

actions: {
    signUp() {
        let username = this.get('username');
        let password = this.get('password');
        let email = this.get('email');
        const credentials = { username, password, email };
        let mutation = Mutation.CREATE_USER;

        return this.get('apollo').mutate({mutation, credentials}, 'createUser').then(() => {
        this.set('username', '');
        this.set('email', '');
        this.set('password', '');
        console.log('user created successfully!');
    }).catch(error => console.log('ERROR!!!!111:', error));

After clicking submit button in a form, I got this message

POST http://localhost:4200/graphql 500 (Internal Server Error)
ServerParseError: Unexpected token < in JSON at position 0

What can I do?