Error after the validation of a form

Hi Franky,
I have created a filter to change the case of the first and last name entered in a membership form. I must have made a mistake because I get the message :
‘Form tampering detected. If you believe you’ve received this message in error please contact the site owner.’
The filter works fine on my local site and the error is only caused on the production site.
This is the filter:

add_filter('eme_insert_person_filter','eme_my_insert_person_function');
function eme_my_insert_person_function($person){
   	$first_name=$person['firstname'];
	$first_name=strtolower($first_name);
	$first_name=ucfirst($first_name);
	$person['firstname']=$first_name;
	$last_name=$person['lastname'];
	$last_name=strtolower($last_name);
	$last_name=ucfirst($last_name);
	$person['lastname']=$last_name;
   return $person;
}

The properties of this membership is to create WP user if not exist.

Can you tell me where my mistake is ?

Frédéric