on my form i got four radio button and i want my user to choose this radio button before they submit it.

what i do:
i create a function named

validateRadio(form)
{
if (form.radiobtt.checked)
{}else
{alert("you must choose one of this radio button");
return false;
}

but with this function, i got error telling me that 'the object does'nt support this....

what wrong and how should i code it the right way

(question from newbie )