OnServerStart not being called in standalone build. Works in editor.

Unity 5.5.2f1
I have implemented an override of OnServerStart in my GameMananager class:

 public class GameManager : NetworkBehaviour
 {
     static GameManager _manager = null;
     public static GameManager Instance { get { return _manager; } }
 
     ShipManager _shipManager;
 
     void Awake()
     {
         if (_manager == null) _manager = this;
         else if(_manager != this) Destroy(gameObject);
 
         DontDestroyOnLoad(gameObject);
 
         _shipManager = GetComponent<ShipManager>();
     }
 
     #region Overrides of NetworkBehaviour
 
     public override void OnStartServer()
     {
         base.OnStartServer();
         Debug.Log("OnStartServer Running...");
         _shipManager.LoadShips();
     }
 
     #endregion
 }

When I run in the editor as "Host", OnStartServer runs and my ships are created.
However, when I build and run in standalone mode, nothing happens. The log does not indicate that OnStartServer was ever called. What am I missing here?