Error in build at AsyncOperation.progress but not in builder

I've come across some odd behaviour. I have an operation where I'm async additive loading a scene, and then later removing the previous scene. Everything runs flawlessly in the editor, but when I build a client I get NullReferenceExceptions, but only at some scenes.

Code (csharp):

1.  
2. IEnumerator unloadLoad()
3.     {
4.         AsyncOperation unload = SceneManager.UnloadSceneAsync("load");
5.         while (true)
6.         {
7.             if(unload == null)
8.             {
9.                 Debug.LogError("Unload was null when attempting to unload 'load' from scene '" + nextScene + "'");
10.                 //Sometimes this will trigger, sometimes not, even if the NullRef below does get thrown. Never thrown when run in editor.
11.             }
12.             if(unload.progress >= 0.9f) //NullRef here. Never thrown when run in editor.
13.             {
14.                 break;
15.             }
16.             yield return null;
17.         }
18.     }
19.  

I'm using Unity 2017.1.1f1 and I've tried a few variations of the script above.
Additionally, the scene does actually get unloaded so nothing is -actually- broken. Just, errors are being thrown.