Search a substring in an array of Strings

I'm trying to search a substring in an array of Strings. I'm using the following code:

 var obstacles = ["Border", "Boundary", "BoundaryFlame"]; 
 var hitFront: RaycastHit;
 if (Physics.Raycast(transform.position, Vector3.forward, hitFront, 1)) {
     for (var i = 0; i < obstacles.length; i++)
     {
         if (obstacles[i].indexOf(hitFront.collider.gameObject.name) > 0)
         {
             //Do something
         }
     }
 }

I'm getting the following error:

MissingMethodException: Method not found: 'System.String.indexOf'.

What am I doing wrong?