seamless-immutable converting my Object to Hash

Question

I am using react+ redux with immutable js. I would like to store an object in the state:

mainFilesUploader = FileUploader({ fileCredentials: action.fileCredentials }, fileUploadType.mainFiles)
formState = formState.set('fileUploaders', {
  mainFiles: mainFilesUploader,
})

Before being put in state the object looks something like this:

FineUploaderS3 {options: {…}, methods: qq.s3.FineUploaderBasic, qq: ƒ}

However at the end of the state... it's just a plain hash:

{options: {…}, methods: {…}, qq: ƒ, merge: ƒ, replace: ƒ, …}

Is there a way I can stop this stupid conversion? Why does it convert my object ?