[jQuery] $("#field") is null

[jQuery] $("#field") is null


Hello,
I have a selectbox with month periods in it.
Here is my code:
$(function(){
            $("#ppsub_ppterm_id").change(function(){
                    var term = this.options[this.selectedIndex].text;
                    if(term == "Eenmalig"){
                        $(".idealtd").show();
                    }else{
                        $(".idealtd").hide();
                        //$("#ppsub_amount option:selected").val('anders');
                    }
            });
    });
<select name="ppsub_ppterm_id" class="ppsub_ppterm_id"
id="ppsub_ppterm_id" style="width: 100px; font-size: 11px;">
                        <option value="M">Maand</option>
                        <option value="K">Kwartaal</option>
                        <option value="H">Halfjaar</option>
                        <option value="J">Jaar</option>
                        <option selected value="E">Eenmalig</option>
                    </select>
But when i load my page i staight away get an error:
$("#ppsub_ppterm_id") is null
Line 17
any idea's?