Uncaught TypeError: dispatch(...).then is not a function

Container component

import { connect } from 'react-redux';
import { signUpUser } from '../actions/userActions';

import Register from '../components/register';

function mapStateToProps(state) {
    return { 
      user: state.user
    };
}

const mapDispatchToProps = (dispatch, ownProps) => {
    return {

    }
}

export default connect(mapStateToProps, mapDispatchToProps)(Register);

Register form

import React, { Component, PropTypes } from 'react';
import { Link } from 'react-router-dom';
import { reduxForm, Field, SubmissionError } from 'redux-form';
import { signUpUser } from '../actions/userActions';

//Client side validation
function validate(values) {
  var errors = {};
  var hasErrors = false;


  return hasErrors && errors;
}

//For any field errors upon submission (i.e. not instant check)
const validateAndSignUpUser = (values, dispatch) => {
    //console.log(values);
    return dispatch(signUpUser(values))
      .then((response) => {
      console.log(response);
    });
};


class SignUpForm extends Component {

  render() {
    const { handleSubmit } = this.props;
    return (
      <div className="col-md-6 col-md-offset-3">
        <h2>Register</h2>
        <form onSubmit={ handleSubmit(validateAndSignUpUser) }>
          <div className ='form-group'>
              <label htmlFor="firstname">Name</label>
              <Field name="firstname" type="text" component= "input"/>
          </div>
          <div className ='form-group'>
              <label htmlFor="username">Username</label>
              <Field name="username" type="text" component= "input"/>
          </div>
          <div className ='form-group'>
              <label htmlFor="password">Password</label>
              <Field name="password" type="text" component= "input"/>
          </div>
          <div className="form-group">
              <button className="btn btn-primary">Register</button>
              <Link to="/" className="btn btn-error"> Cancel </Link>
          </div>
      </form>
     </div>
    )
  }
}

export default reduxForm({
  form: 'SignUpForm', // a unique identifier for this form
  validate
})(SignUpForm)

Actions

import axios from 'axios';

export function signUpUser(user) {
  console.log(user);

  const url = `https://jsonplaceholder.typicode.com/posts`
  const request = axios.get(url); 

  return {
    type: 'Register_User',
    payload: request
  };
}

When I submit this form I am getting following error. This app uses thunk, setup form reducer in combined reducer. Where am I going wrong? I am new to redux-form and thunk

Uncaught TypeError: dispatch(...).then is not a function