Multiplayer Custom Object Spawning (ClientScene.RegisterSpawnHandler) Out Of Sync Error

I am trying to spawn an object across the network.
This object is defined by configuration data loaded at runtime, so as far as I can tell I cannot use the ClientScene.RegisterPrefab() -> NetworkServer.Spawn() protocol.

As an alternative the Unity Scripting API suggests using ClientScene.RegisterSpawnHandler().
However, in attempting to use it within some stripped-down testing code I keep being stopped by the following error:

 InvalidOperationException: out of sync
 System.Collections.Generic.Dictionary`2+Enumerator[UnityEngine.Networking.NetworkInstanceId,UnityEngine.Networking.NetworkIdentity].VerifyState () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:912)
 System.Collections.Generic.Dictionary`2+Enumerator[UnityEngine.Networking.NetworkInstanceId,UnityEngine.Networking.NetworkIdentity].MoveNext () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:835)
 System.Collections.Generic.Dictionary`2+ValueCollection+Enumerator[UnityEngine.Networking.NetworkInstanceId,UnityEngine.Networking.NetworkIdentity].MoveNext () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:1145)
 UnityEngine.Networking.NetworkServer.ActivateLocalClientScene () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkServer.cs:361)
 UnityEngine.Networking.ClientScene.ConnectLocalServer () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/ClientScene.cs:299)
 UnityEngine.Networking.NetworkManager.ConnectLocalClient () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:538)
 UnityEngine.Networking.NetworkManager.StartHost () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:526)
 UnityEngine.Networking.NetworkManagerHUD.OnGUI () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManagerHUD.cs:80)

The code I am using is as follows (the error occurs on the NetworkServer.Spawn line):

 // Runs on Local Host
 private void SpawnLocation(GameObject type, Vector2 pos, string ownerTag, string controllerTag, int level)
 {
     GameObject testObj = new GameObject();
     testObj.AddComponent<NetworkIdentity>();
 
     NetworkHash128 testAssetId = NetworkHash128.Parse("e2656f");
     
     RpcRegisterCustomLocationSpawner();
 
     NetworkServer.Spawn(testObj, testAssetId);
 }
 
 [ClientRpc]
 private void RpcRegisterCustomLocationSpawner()
 {
     NetworkHash128 testAssetId = NetworkHash128.Parse("e2656f");
     ClientScene.RegisterSpawnHandler(testAssetId, SpawnCustomLocation, UnspawnCustomLocation);
 }
 
 private GameObject SpawnCustomLocation(Vector3 position, NetworkHash128 testAssetId)
 {
     GameObject testObj = new GameObject();
     testObj.AddComponent<NetworkIdentity>();
 
     return testObj;
 }
 
 private void UnspawnCustomLocation(GameObject obj)
 {
     obj.SetActive(false);
 }