NaN exception when spawning soldiers

Im getting a strange error randomly when they spawn, what is "transform.position assign attempt for 'spawnSoldier(Clone)' is not valid. Input position is {NaN, 0.262370.NaN}" i have no idea how to fix this :S

Heres my code

 #pragma strict
 
 var prefab:GameObject; 
 var soldierMax:int;
 var soldierCount:int; 
 private var respawnPoints:GameObject[];
 var maxSpawnDelay:int;
 var soldiersKilled:int;
 
 function Start () {
      respawnPoints = GameObject.FindGameObjectsWithTag("SoldierSpawn");
      soldierCount = GameObject.FindGameObjectsWithTag("Soldier").length;    
 }
 
 function Update () {
     if(soldierCount < soldierMax) {
         respawnSoldier();        
     }        
 }
 
 function respawnSoldier() {
     while (soldierCount < soldierMax) {
         Instantiate(prefab,respawnPoints[Random.Range(0,respawnPoints.Length)].transform.position,Quaternion.identity);
         soldierCount++;
         yield WaitForSeconds(Random.Range(3,maxSpawnDelay));
     }
 }