Interfaces Custom Properties URI is NULL

I'm trying to use the InterfacesCustomProperties table's URI to update interface custom properties with SWIS. I've done this successfully with Node custom properties, but I'm running into an issue with using the URI for interfacescustomproperties. Every URI is NULL, and I haven't seen this happen before. I see valid URIs for node custom properties, nodes, and interfaces, but not interface custom properties. Is there something that has to happen before the URI is generated for interface custom properties? Has anyone seen this happen to them? If you have, did you get URIs to generate for interface custom properties? Is there some reason why the URI is NULL in this table for interface custom properties?

thanks!