I am not sure if this is the right topic to post a bug but i could not find a better place for this; apologies if it is not.
I like to report that the web service core_course_update_courses function returns error if the forcetheme or lang values are empty string. This is not spouse to happen because under course settings you could set any of these two fields to be the default value (which sets an empty string in the database). For example you could set the Force theme or Force language to be Do not force (which sets an empty string in the database).
Currently you can't set any of these two fields to Do not force value through the web service function mentioned above.