Calling a MySql function

I have created a function in mysql called NextVal that requires a string parameter and returns an integer.
I tried the following code:

Code:

public function next_value() {
    $ci =& get_instance();
    $sql = 'SELECT nextval(?) as receipt_id';
    $query = $ci->db->query( $sql, array( $this->sequence_name ) );
    return $query->row()->receipt_id;
}

Codeigniter logs the following error:
ERROR - 2019-04-28 19:16:55 --> Query error:  - Invalid query: SELECT nextval('receipt_id_seq') as receipt_id

However, if I paste and run the select statement shown in the error in MySql Workbench it returns the expected integer value under the column name receipt_id

The full error is:
Query error: execute command denied to user 'my_app_username'@localhost for routine 'my_schema.nextval' - Invalid query:

I have tried the following but no joy:
grant execute on db.* to 'my_app_username'@localhost;
FLUSH PRIVILEGES;