checkbox 选中按钮可用，未选中按钮不可用

<div class="checkbox">
<label>
<input type="checkbox" checked="checked" onclick="terms()">我已阅读并接受<a  data-target="#termsModal" data-toggle="modal">《服务条款》</a>
</label>
</div>
<div class="row">
<div class="col-md-12">
<input type="button" id="btn_reg" value="注册" class="btn btn-primary pull-right mb-xl" onclick="regPotentialUser()"/>
</div>
</div>

function terms(){
if(!$("input[type='checkbox']").is(':checked'))
{
$("#btn_reg").attr("disabled", true).css("background-color","#666");
}else{
$("#btn_reg").attr("disabled", false).css("background-color","#f60");
}

}