diff --git a/tests/build.yml b/tests/build.yml
index 3436d25a..e06327cd 100644
--- a/tests/build.yml
+++ b/tests/build.yml
@@ -31,7 +31,7 @@ services:
     build: ../optional/radicale
 
   traefik-certdumper:
-    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}traefik-certdumper:${MAILU_VERSION:-local}
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}traefik-certdumper:${MAILU_VERSION:-local}
     build: ../optional/traefik-certdumper
 
   admin: