diff --git a/docker/build_debian.sh b/docker/build_debian.sh
index 801ff45471..1d0be42cfc 100644
--- a/docker/build_debian.sh
+++ b/docker/build_debian.sh
@@ -5,7 +5,7 @@
 set -ex
 
 # Get the codename from distro env
-DIST=`cut -d ':' -f2 <<< $distro`
+DIST=`cut -d ':' -f2 <<< ${distro:?}`
 
 # we get a read-only copy of the source: make a writeable copy
 cp -aT /synapse/source /synapse/build