WebGL error: "Could not produce class with ID 0"

Hi, I am working on a WebGL project with asset bundles. Everything seems to work but I am getting the following error even when I disable 'Strip Engine Code'. Any thoughts? I don't find any reference to a class with ID 0.> blob:http://localhost:8000/d89a9c3f-9b5c-49a3-8e70-0ca3b59f0227:8336 Could not produce class with ID 0.> This could be caused by a class being stripped from the build even though it is needed. Try disabling 'Strip Engine Code' in Player Settings.>> (Filename: /Users/builduser/buildslave/unity/build/Runtime/Serialize/PersistentManager.cpp Line: 1323)