Crouch jump bug.

Hi,i have crouch script it works perfect,i have fixed the un-crouching ceiling bug too,but when i crouch under the object,everything is perfect,but when i jump,i get a bit through that object..try yourself,please help me to fix this bug.

     var controller : CharacterController; //ADD in Inspector
         var isCrouched : boolean = false;
         var checkCeiling : boolean;
        
         function Update () {
             if(Input.GetKey("c")){
                 if(!isCrouched) crouch();
             }else{
                if(isCrouched && !checkCeiling) stand();
             }
            
             if(isCrouched){
                 if(Physics.Raycast (transform.position, Vector3.up, 2.0)){
                     checkCeiling = true;   
                 }else{
                     checkCeiling = false;
                 }
             }      
         }
        
         function crouch(){
             controller.height /= 2;
             isCrouched = true;
         }
      
         function stand(){
             gameObject.transform.position.y += controller.height/2;
             controller.height *= 2;
             isCrouched = false;
         }