[] Error using Connected Variable Driver

In order to show a specific string on a dashboard, I decided to use a device with the ā€œConnector Variableā€ driver. I have setup a Rule to populate the deviceā€™s value and it works. However, I get the following error in my logs:

This is the device I am populating:

Here is the Rule, with an arrow on the line that I think is triggering the error (The device name is ā€œunderscoreā€ (or ā€œ_ā€) not to interfere with the other button underneath it on the dashboard).

Curious if anyone has seen a similar error? I suspect that Connector Variables were not intended for this purpose, hence the errorā€¦