Nefunguje ověření checkboxu

<form action="/none/odeslano.php" method="post" name="auto">
         <input type="checkbox" name="znacka[]" onClick="" value="Alfa"> Alfa
          <input type="checkbox" name="znacka[]" onClick="" value="BMW"> BMW
           <input type="checkbox" name="znacka[]" onClick="" value="Citroen"> Citroen
            <input type="checkbox" name="znacka[]" onClick="" value="Ford"> Ford
            <input type="checkbox" name="znacka[]" onClick="" value="Honda"> Honda
            <input type="checkbox" name="znacka[]" onClick="" value="Jaguar"> Jaguar
            <input type="button" value="Overit" onClick="return showBox(this.form);">       

         </form>

  <script type="text/javascript">
         function showBox(frm) {
          var zprava = "";
             for(i=0 ; i < frm.znacka.length ; i++) 
              if(frm.znacka[i].checked) {
                zprava = zprava + frm.znacka[i].value + "\n";
                  } 
              alert(zprava);  
              if(zprava == "") {
               alert("Prazdny retezec");
                 }
               else {
                alert("Neco je vybrano");
                  }
              }
    </script>