I have an error from a script from the standard assets...

I get an error that says the name space "ThirdPersonCharacter" could not be found any Ideas how to fix this?.. the script...

 namespace UnityStandardAssets.Characters.ThirdPerson
 {
     [RequireComponent(typeof (NavMeshAgent))]
     [RequireComponent(typeof (ThirdPersonCharacter))]
     public class AICharacterControl : MonoBehaviour
     {
         public NavMeshAgent agent { get; private set; }             // the navmesh agent required for the path finding
         public ThirdPersonCharacter character { get; private set; } // the character we are controlling
         public Transform target;                                    // target to aim for
 
 
         private void Start()
         {
             // get the components on the object we need ( should not be null due to require component so no need to check )
             agent = GetComponentInChildren<NavMeshAgent>();
             character = GetComponent<ThirdPersonCharacter>();
 
             agent.updateRotation = false;
             agent.updatePosition = true;
         }
 
 
         private void Update()
         {
             if (target != null)
                 agent.SetDestination(target.position);
 
             if (agent.remainingDistance > agent.stoppingDistance)
                 character.Move(agent.desiredVelocity, false, false);
             else
                 character.Move(Vector3.zero, false, false);
         }
 
 
         public void SetTarget(Transform target)
         {
             this.target = target;
         }
     }
 }