checkbox name[] to jquery $.ajax

Why my checkboxes:checked don't return nothing? When I check the items stated below, and then submit the form (with preventDefault()), they have an 'error_class' added, instead of having it removed. While the rest of my other inputs behave normally if they're not empty.

<label class="label" id="label_contrat">Type de contrat recherché</label>     
   <input type="checkbox" name="contrat[]" value="CDI" class="contrat_checkbox" tabindex="113">         <label for="CDI"  class="tag_contrat">CDI</label> 
   <input type="checkbox" name="contrat[]" value="CDD" class="contrat_checkbox" tabindex="114">         <label for="CDD"  class="tag_contrat">CDD</label> 
   <input type="checkbox" name="contrat[]" value="Interim" class="contrat_checkbox" tabindex="115">     <label for="Interim"  class="tag_contrat">Interim</label> <br />
   <input type="checkbox" name="contrat[]" value="Saisonnier" id='saison_c' class="contrat_checkbox" tabindex="116">  <label for="Saisonnier" class="tag_contrat">Emploi Saisonnier</label> 
   <input type="checkbox" name="contrat[]" value="Autres" class="contrat_checkbox" tabindex="117">      <label for="Autres"  class="tag_contrat">Autres</label> 



<input type="checkbox" name="cgu_inscription" id="cgu_inscription" tabindex="140"/>
 <label class="label" id='label_cgu'>J'accepte les <a href="#" id="CGU_link">CGU</a></label>



     /*  var contrat = []
    $("input[name='contrat[]']:checked").each(function ()
    {
        contrat.push(parseInt($(this).val()));
    });*/

    var contrat = { 'contrat[]' : []};
     $("input[name='contrat[]']:checked").each(function() {
       contrat['contrat[]'].push($(this).val());
     });

    var cgu_inscription = $("#cgu_inscription:checked").val();


$.ajax({ 
type : 'POST',
 url : "process.php",
 data: {myOtherValues:myOtherValues, contrat:contrat,  cgu_inscription:cgu_inscription}
 }, dataType:"json"
     })
.done(function(data) {  }
// etc