Can't use declared variable to find nearest Object

Hello everybody,

I want to "respawn" a player to the race track when he falls off the map and touches a trigger volume, this is my code :

 GameObject GetClosestObject(string tag) 
     {
         GameObject ClosestObject;
         GameObject[] gos = GameObject.FindGameObjectsWithTag(tag);
 
         float distance = Mathf.Infinity;
 
         foreach (GameObject go in gos) 
         {
             Vector3 diff = go.transform.position - transform.position;
             float curDistance = diff.sqrMagnitude;
 
             if (curDistance < distance) 
             {
                 ClosestObject = go;
                 distance = curDistance;
             }
         }
 
         return ClosestObject;
     }

But I get this error in the last line :

error CS0165: Use of unassigned local variable `ClosestObject'

Anyone knows what I'm doing wrong?