Reference a prefab animationClip in standAlone build

Hi.

I am referencing a prefab's animationClip in a script. It works fine in the editor, but throws this error in the standalone windows build.

MissingMethodException: Cannot find the requested method. at (wrapper managed-to-native) UnityEngine.Motion:get_averageDuration () at PrefabScript.get_animLength () [0x00000] in D:\Workspaces\Unity\Test Bed\Assets\Prefab Referencing\PrefabScript.cs:12 at Main.Start () [0x00000] in D:\Workspaces\Unity\Test Bed\Assets\Prefab Referencing\Main.cs:11 (Filename: D:/Workspaces/Unity/Test Bed/Assets/Prefab Referencing/PrefabScript.cs Line: 12)

I'm using the following scripts

 using UnityEngine;
 using System.Collections;
 
 public class Main : MonoBehaviour {
 
     public PrefabScript prefab;
 
     // Use this for initialization
     void Start ()
     {
         Debug.Log(prefab.animLength.ToString());
     }
 }
 

 using UnityEngine;
 using System.Collections;
 
 public class PrefabScript : MonoBehaviour
 {
     public AnimationClip anAnimation;
 
     public float animLength
     {
         get
         {
             return anAnimation.averageDuration;
         }
     }
 }
 

All references and public variables are properly set. Can anyone shed any light on why this works in the editor and not on a stand alone windows build?