this.setState within callback unmounts

Really confused here. I updated to React Router 4 which required a few changes and now when my registration form errors on the server side it returns the console error:

setState(…): Can only update a mounted or mounting component. This
usually means you called setState() on an unmounted component. This is
a no-op. Please check the code for the RegisterForm component.

What’s really confusing is if I run this.setState({ errors: {createUserError: "Test error" }}); outside of Accounts.createUser function I don’t get the consolde error.

Any suggestions???

handleSubmit(event) {

    this.setState({errors: {} }, function() {
      var data = {
        password: this.state.password

        Accounts.createUser(data, (error) => {   // This arrow function preserves this
        if(error) {
          this.setState({ errors: {createUserError: error.reason }});