Wrong value when method called from a button

This has been driving me nuts - I've got a public reference that is assigned in the editor. Normally, it's not null and behaves correctly. However, when the method "okay" is called by pressing a button, the reference is null. What is going on? Is this some editor magic that I'm not aware of? I'm using 5.1.1f1; was this a known issue with this version?

Thanks so much for any help!

 using UnityStandardAssets.Characters.FirstPerson;
 
 public class OptionsData : MonoBehaviour {
 
     public FirstPersonController mouseInfo;
 
     void Update()
     {
         Debug.Log (mouseInfo == null);
         okay ();
     }
 
     public void okay()
     {
         Debug.Log (mouseInfo == null);
     }
 }