yep

Gameboard.ShogiUI.Sockets/Controllers/GameController.cs

@@ -100,7 +100,7 @@ namespace Gameboard.ShogiUI.Sockets.Controllers
           PlayerName = user.Name,
           Move = moveModel.ToServiceModel()
         }, session.Player1, session.Player2);
-        return Ok();
+        return Created(string.Empty, null);
       }
       throw new InvalidOperationException("Illegal move.");
     }

Gameboard.ShogiUI.Sockets/Managers/GameboardManager.cs

@@ -34,11 +34,11 @@ namespace Gameboard.ShogiUI.Sockets.Managers
       while (count < MaxTries)
       {
         count++;
-        var clientId = $"Guest-{Guid.NewGuid()}";
-        var isCreated = await repository.CreateGuestUser(clientId, webSessionId);
+        var userName = $"Guest-{Guid.NewGuid()}";
+        var isCreated = await repository.CreateUser(new User(userName, webSessionId));
         if (isCreated)
         {
-          return clientId;
+          return userName;
         }
       }
       throw new OperationCanceledException($"Failed to create guest user after {count} tries.");

Gameboard.ShogiUI.Sockets/Repositories/GameboardRepository.cs

@@ -241,8 +241,12 @@ namespace Gameboard.ShogiUI.Sockets.Repositories
         logger.LogError(new InvalidOperationException(result.warning), result.warning);
         return null;
       }
-
-      return new Models.User(result.docs.Single().Name);
+      var userDocument = result.docs.SingleOrDefault();
+      if (userDocument != null)
+      {
+        return new Models.User(userDocument.Name);
+      }
+      return null;
     }
   }
 }