Notice: Trying to get property of non-object in i18n_langcode()

When the Webform Protected Downloads module is enabled on my multilingual website (which uses Entity Translation and Domain Access), I'm getting this notice after submitting a form with protected downloads (it's also visible for anonymous visitors):

Notice: Trying to get property of non-object in i18n_langcode() (line 229 of /sites/all/modules/contrib/i18n/i18n.module).

When I disable the module, the notice is gone, but then my downloads are no longer protected ofcourse.

What could be the cause of this issue please?