How to solve "Invalid Taxonomy", displays Array array on WordPress page?

Question

When I var_dump($term_list);

$term_list = wp_get_post_terms($post->ID, 'functiongroups', array("fields" => "names")); 

foreach ($term_list as $term) { $i++;
         echo '.$term.';
         echo ($i > 0 && !($i % 2 == 0)) ? ', ' : '';
};

I'm getting this:

WP_Error Object ([errors] => Array ([invalid_taxonomy] => Array ([0]
=> Invalid Taxonomy)) [error_data] => Array ())

, and displays Array array on WordPress page and not the expected taxonomy.

Answer 1

The name of the taxonomy didn't exist (i.e. a taxonomy created using a plugin like advanced-custom-fields or any other plugin using taxonomies) or was misspelled when using wp_get_post_terms();. (In my case 'functiongroups' didn't exist);

Note that the taxonomy could be added programmatically as well, not only by using a plugin:

function functiongroups_init() {
    // create a new taxonomy
    register_taxonomy(
        'functiongroups',
        'post',
        array(
            'label' => __( 'Functiongroups' ),
            ...
        )
    );
}
add_action( 'init', 'functiongroups_init' );