diff --git a/vagrant/provision/assets/build-tails b/vagrant/provision/assets/build-tails
index 7ead5cdb35b49b1a0d49a886428aeb4da94702f9..1a0d08307c2a5f26a62c9771287ba90a77aba444 100755
--- a/vagrant/provision/assets/build-tails
+++ b/vagrant/provision/assets/build-tails
@@ -138,7 +138,7 @@ if [ "${TAILS_PROXY_TYPE}" = "vmproxy" ]; then
     # of it is used there is 5G before each build, which should be
     # enough for any build, even if we have to download a complete set
     # of new packages for a new Debian release.
-    /usr/lib/apt-cacher-ng/acngtool shrink 10G -f || \
+    as_root_do /usr/lib/apt-cacher-ng/acngtool shrink 10G -f || \
         echo "The clean-up of apt-cacher-ng's cache failed: this is" \
              "not fatal and most likely just means that some disk" \
              "space could not be reclaimed -- in order to fix that" \