EntityLink segment not allowed. Contact your administrator to enable it.

When using the current version of the helper to create a post in a community, with a reference to a file, I get the error "EntityLink segment not allowed. Contact your administrator to enable it". This does not seem to happen when I create posts internally (eg: when the networkId is null). Has anyone else run into this, and if so, is it actually possible to "enable it" as the error message indicates, or is this a limitation of posting into a community?