is_multisite() via 'show_on_cb'

Hi.

I’m trying to get an individual meta field to display if an installation is multisite enabled using the function is_multisite().

It doesn’t seem to be working for some reason, and my field is showing regardless. Here is the code:

$cmb->add_field( array(
            'name' => __( 'Global Welcome Message', 'cm_lang' ),
            'desc' => __( 'This will act as the default Welcome Message (displayed on each site\'s homepage). This can be overridden by adding a Welcome Message on an individual site\'s Site Options.', 'cm_lang' ),
            'id'   => 'global_welcome_message',
            'type' => 'wysiwyg',
            'show_on_cb' => cmb_is_multisite()
        ) );

And the callback function:

function cmb_is_multisite() {
    if ( is_multisite() ) {
        return true;
    }
}

I wrote the callback function since using plain is_multisite() didn’t work.

Thanks

https://www.ads-software.com/plugins/cmb2/