Error: “undefined function wp_insert_link()” – I’m unable to find the cause

I have a my own custom plugin that looks like this:

<?php
function fwp_mycustomfunction ( $settings ) {
    // ... code 
    $linkid = $settings['link/id'];
    $bookmark = get_bookmark($linkid, ARRAY_A);
    // ... code that modifies bookmark
    $result = wp_insert_link($bookmark, true);
    // ... code
}
add_action('feedwordpress_check_feed_complete', 'fwp_mycustomfunction', 1000);
?>

I get this in error.log

2018/01/25 08:04:35 [error] 23690#0: *12022 FastCGI sent in stderr: “PHP message: PHP Fatal error: Call to undefined function wp_insert_link()

What could be wrong and how do I solve it?