Creating a plugin, getting Warning: call_user_func_array() expects parameter 1

I’m trying to put together my own little plugin. Everything went fine and I decided I’d try breaking the code up into several different files. Bear with me as I’m fairly new to making plugins, I’ve only made very simple personal ones before.

I started getting this error after dividing it into multiple files, is it something obvious I’ve missed? I looked at what other plugins did to see how it’s done (or how I think it is).

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘create_fvmenu’ not found or invalid function name in /Users/name/Sites/wordpress/wp-includes/plugin.php on line 405

fvshop.php:

<?php
/*
Plugin Name: FVshop
Version: 0.1
*/

define( 'FVSHOP_URL', WP_PLUGIN_URL.'/fvshop' );
define( 'FVSHOP_DIR', WP_PLUGIN_URL.'/fvshop' );

if (is_admin()) {
    require_once( FVSHOP_DIR . '/includes/fvshop-admin-core.php');
}

if (is_admin()) {
    add_action('admin_init', 'create_fvmenu');
}

/includes/fvshop-admin-core.php:

<?php
function create_fvmenu() {
    add_action('admin_menu', 'fvshop_menu');

    function fvshop_menu() {

        $args = array(
            'page_title'    =>  'FV Shop',
            'menu_title'    =>  'FV Shop',
            'capability'    =>  'manage_options',
            'menu_slug'     =>  'fv_shop',
            'function'      =>  '',
            'icon_url'      =>  '',
            'position'      =>  ''
        );
        add_menu_page('FV Shop', 'FV Shop', 'manage_options', 'fv_shop', 'fv_shop_options', '', '3');

        add_submenu_page( 'fv_shop', 'Options', 'Options', 'manage_options', 'opt', 'fv_shop_options');
    //    add_options_page('FV Shop Options', 'FV Shop', 'manage_options', 'fvshopbutik', 'fv_shop_options');
    }

    function fv_shop_options() {
        if (!current_user_can('manage_options')) {
            wp_die(__('You do not have sufficient permissions to access this page.') );
        }
        echo '<div class="wrap">';
        echo '<p>FVshoppens options.</p>';
        echo '</div>';
    }
}

Doesn’t really matter what kind of function I put in the second file, I still get the same error.

Could someone shine some light on what I’m doing wrong, or what it is that could be happening?