Unknow user property rs_inlinerelationship

We’ve just upgraded from 5.7 to 6.5.2 and everything seems to be working with the exception of adding inline links through ephox.

The link can be added, but when trying to insert the new content item, an error occurs:

java.lang.IllegalArgumentException: Unknown user property name: ‘rs_inlinerelationship’

Anyone know how I can resolve this?