Why I get this Debug error: 'fileLength == processed' when calling CreatePrefab?

I'm building an Editor script to compose a GameObject given a set of Assets. The final objective is to create a prefab to be used by the rest of the team.

It's working fine except by a minor caveat: when I call PrefabUtility.CreatePrefab(prefabPath, myGameObject); it shows this message as an Error in the Console Log:

 fileLength == processed
 UnityEditor.PrefabUtility:CreatePrefab(String, GameObject)
 SP.CharacterBuilder.Tests.Unit.PrefabCreationTests:CreatePrefabFromMesh() (at Assets/Packages/SP/SP/CharacterBuilder/Editor/Tests/Unit/PrefabCreationTests.cs:58)
 System.Reflection.MethodBase:Invoke(Object, Object[])
 ...
 ...
 

Note: I'm running the code using the Unity3d Unit Test Runner, so the stack trace is partial

However it seems to work fine. The prefab file is created in the correct path, the prefab structure is the same as I've defined in the source GameObject, I can Instantiate the prefab, either by code or just drag&dropping the prefab file... etc.

Anyone knows why is this weird error showing?

Running Unity 4.3.3f1