Why does GetAssetPreview return null for model?

Below is my very simple code. I am trying to get a preview image like Unity uses in its selection menus in the editor. Instead, it returns null.

Are the certain settings that must be set on the model to be able to generate a preview?

        public Texture2D PreviewImage {
         get {
             if (tilePreviewImage==null){
                 Debug.Log("Model: "+_model);
                 tilePreviewImage = AssetPreview.GetAssetPreview(_model);
             }
             return tilePreviewImage;
         }
     }

I am trying to call it during an OnGUI if that makes a difference.

Also the model in question was loaded from a bundle.