Form Validation Problem .. Null Object Error

This is a simple validation code i use only for making sure they check a radio button. The problem is i get this "radioname.checked" is a null object. Please help.
function validate(formObj)
{
if (formObj.radioname1.checked) {
alert("Please select ");
formObj.radioname1.focus();
return false;
}
if (!formObj.radioname5.checked) {
alert("P");
formObj.radioname5.focus();
return false;
}
if (!formObj.radioname6.checked) {
alert("");
formObj.radioname6.focus();
return false;
}
alert("WTF");
return true;
}

<form action+"mailer.exe" onsubmit= "return validate(this)>
<input name="radioname" type="radio" id="radioname" value="I'll be there">
<input type="image" name=submit src="...">
</form>

And I get a weird null object error please help

That's because there is no radiobutton in your form with that name. You only have one radiobutton called name="radioname". Or...did you just omit them from the example?