IndexOutOfRangeException In InterpolateFast

[UsernameHed]

Hello, I have sometime a little IndexOutOfRangeException when i am initalizing things, I have an error at the return of this function:

Code:

public Vector3 InterpolateFast(float localF)
       {
           float frag;
           int idx = getApproximationIndexINTERNAL(localF, out frag);
           int idx2 = Mathf.Min(Approximation.Length - 1, idx + 1);
           return (Vector3.LerpUnclamped(Approximation[idx], Approximation[idx2], frag));
       }

I have done this extra test to make it work:

Code:

if (idx < 0 || idx >= Approximation.Length || idx2 < 0 || idx2 >= Approximation.Length)
           {
               return (Vector3.zero);
           }

My 2 questions ares: In what condition that peace of code can return an idx || idx2 invalid, and secondly, is my patch with return Vector3.zero is safe or not ? 

Thanks