Trying to check for form errors

I’m using the code below. The script thinks the checkbox is checked. It is not checked.

if (!document.f1.SK_1.checked && document.f1.SK_Number1.value == "") {
alert("Please enter your profile link");
document.f1.SK_Number1.focus();
return false;
}

if (!document.f1.SK_2.checked && document.f1.SK_Number2.value == "") {
alert("Please enter your profile link");
document.f1.SK_Number2.focus();
return false;
}

<input class="selectRadioButtons" type="checkbox" name="SK_1"  value="$provlist[0]" > <BR>
<input class="selectClass" type="text" name="SK_Number1" value="" placeholder="$provlist[0] Profile Link">