diff --git a/functions b/functions
index 4981926a7..91edb9473 100644
--- a/functions
+++ b/functions
@@ -17,8 +17,8 @@ install_external() {
 #
 # - ``PUPPETFILE_DIR`` must be set to Puppet modules directory
 # - ``SCRIPT_DIR`` must be set to script path
-# - ``ZUUL_REF`` must be set to Zuul ref
-# - ``ZUUL_BRANCH`` must be set to Zuul branch
+# - ``ZUUL_REF`` must be set to Zuul ref. Fallback to 'None'.
+# - ``ZUUL_BRANCH`` must be set to Zuul branch. Fallback to 'master'.
 # - ``ZUUL_URL`` must be set to Zuul URL
 install_openstack() {
   cat > clonemap.yaml <<EOF
@@ -27,6 +27,10 @@ clonemap:
     dest: '$PUPPETFILE_DIR/\2'
 EOF
 
+  # Periodic jobs run without ref on master
+  ZUUL_REF=${ZUUL_REF:-None}
+  ZUUL_BRANCH=${ZUUL_BRANCH:-master}
+
   local project_names=$(awk '{ if ($1 == ":git") print $3 }' \
     ${SCRIPT_DIR}/Puppetfile0 | tr -d "'," | cut -d '/' -f 4- | xargs
   )