email-ext cannot expand ${FAVORITE_USERS_EMAIL} macro in pipeline

When passing the ${FAVORITE_USERS_EMAIL} macro in the 'to:' line of a pipeline call to email-ext, jenkins errors with: "FAVORITE_USERS_EMAIL" is not supported in this context. 

I also cannot expand the ${FAVORITE_USERS_EMAIL} token using the 'tk' step to get the list of users who have favorited a job and include them in the email. The same error message is thrown.