diff --git a/scripts/compute-tags.sh b/scripts/compute-tags.sh
index ac139b0d1a7..342e3c70b3f 100644
--- a/scripts/compute-tags.sh
+++ b/scripts/compute-tags.sh
@@ -5,6 +5,7 @@
 set -exu
 
 BASE_BUILD_IMAGE=$1
+BRANCH=${BRANCH:?'expecting BRANCH env var'}
 
 ## if we are on a release tag, let's extract the version number
 ## the other possible value, currently, is 'master' (or another branch name)