diff --git a/lib/gcp_ci.sh b/lib/gcp_ci.sh
index db86b711..b5f5a44a 100755
--- a/lib/gcp_ci.sh
+++ b/lib/gcp_ci.sh
@@ -82,7 +82,8 @@ printenv(){
 # do not use the same serviceaccount with permissions across projects, you can cross contaminate and make mistakes, deploy the wrong environment etc.
 gcp_login(){
     local credentials_json="${1:-}"
-    if ! [ -f "$credentials_json" ]; then
+    if [ -z "$credentials_json" ] ||
+       ! [ -f "$credentials_json" ]; then
         # XXX: it's hard to copy the contents of this around so it's easiest to do via:
         #
         #   base64 credentials.json | pbcopy