call_user_func_array() expects parameter 1 to be a valid callback

Hi, after searching a lot in the net, I’ve could not find any answer to my problem. I’m using nginx in debian and in the error log I’m getting the following error message:

FastCGI sent in stderr: “PHP message: PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘register_shortcodes’ not found or invalid function name in /wp-includes/plugin.php on line 406” while reading response header from upstream, client: *, server: xxxxxxxx, request: “GET / HTTP/1.1”, upstream: “fastcgi://127.0.0.1:9000”, host: “xxxxxxxxxxxx”

I have some shortcodes in my function.php:

add_shortcode(‘siguenos’, ‘follow_us’);
add_shortcode(‘referencias’, ‘show_references’);
add_action( ‘init’, ‘register_shortcodes’);

Could you please tell me what is happening?

Best regards