PLSQL: using subquery in if-statement error PLS-00405

I'm getting error pls-00405 when I try to run this code:

BEGIN
  IF :P10_KAART_CODE IN (SELECT KAART_CODE FROM CADEAUKAART) THEN
     RETURN TRUE;
  ELSE
    RETURN FALSE;
  END IF;
END;

There are some similar questions about this but couldn't find a solution for this simple code. Is there another way of writing this without facing a error?