Site Collection Policy error when trying to add a site column to a label - "The label reference, DCO Prefix, could not be found."

I have been trying to create a Site Collection Policy and add a label to this policy. I started by creating the fields that I want on the label as site columns in the site collection. Then I entered the label format as follows:

ISO 9001 QMS \nTitle:{Title} \nDCO: {DCO Prefix}-{DCO Number} \nEffective Date:{Effective Date}

The site columns are DCO Prefix, DCO Number, and Effective Date. Title is also the system defined site column Title.

When I Ok to save the policy I get the error message: The label reference, DCO Prefix, could not be found.
If I remove the values the site columns are not system defined (everything other than {Title}) I can save the label.

Is this the expected behavior for a Site Collection Policy? Am I doing something wrong when defining the label format? If so how do I correct it? Is there another way to create a label policy at the Site Collection level that uses a predefined field?