search filter function

I’m trying to write a function for my theme’s functions.php that will exclude specific custom post types from the WordPress search.

Example: this excludes the custom post type, however it also excludes all results from the search.

function SearchFilter($query) {
     if ($query->is_search) {
        $query->set('post_type', '-customposttype');
     }
     return $query;
}
add_filter('pre_get_posts','SearchFilter');

If the – is removed, the search only returns the custom post type.

When I use the almost identical filter to remove posts in a specific category no problems.

function SearchFilter($query) {
     if ($query->is_search) {
        $query->set('cat','-4') ;
     }
     return $query;
}
add_filter('pre_get_posts','SearchFilter');

Any ideas on what I’m doing wrong?