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

[jjsanders]

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?