Getting a data type error and can't work out why

LabVIEW NXG 5.0

I have a simple bit of code that reads a tag and writes a new value to it, but it fails on a data type error at the Read Tag Node and Write Tag Node:

"All Notifiers" is a gtype: a cluster containing a "Reference" to a Notifier.

The value of the nodes:

Wire definition, for wire at Probe 1/3:

Wire Definition for wire at Probe 2/4:

The Tag Definition:

And the actual error:

I can't work out why it's an invalid data type.  I've done something similar but with Event and Message Queue types - exactly the same setup - and they are fine.  

Has anyone got any idea why or how I can debug further?