Use get_userdata() from inside an Ajax-called function

Hi !

I try launching a function from WordPress core from a function that is triggered through Ajax and it seems the core function is not available. Here’s an extract of my code. Any idea ?

add_action( 'wp_ajax_ajax_action', 'getPositionDetails' );
add_action( 'wp_ajax_nopriv_ajax_action', 'getPositionDetails' );

function getPositionDetails() {
// .....
	foreach( $membersHere as $member ) {
		get_userdata($member->user_id)
	}
// .....
}

Thanks ! ??
Yann