Using value set in build script in a Failure Conditions Failure message

I have configured a build script to echo the following before it exits:

echo "##teamcity[setParameter name='env.teamcity_build_failure_message' value='${FAILED_COUNT} of ${COUNT} images failed to build']"

I have then configured a failure condition with the failure message set to the following:

%env.teamcity_build_failure_message%

When I ‘Test on finish build’ I can see the following:

When I save the Failure Condition I see that the Parameters menu item has an error:

If I try to start a build I get an error saying “There are no idle compatible agents which can run this build” and can see the following:

If I then set a parameter value this build will run but will just show whatever value I set as the build status.

Can this be made to work? What am I missing?