diff --git a/wud/stack.env b/wud/stack.env
index a6d59aa..14efc43 100644
--- a/wud/stack.env
+++ b/wud/stack.env
@@ -18,7 +18,7 @@ WUD_TRIGGER_GOTIFY_EXTERNAL_URL=https://gotify.sectorq.eu/
 
 WUD_TRIGGER_GOTIFY_EXTERNAL_MODE=batch
 
-WUD_TRIGGER_DOCKER_UPDATE_PRUNE
+WUD_TRIGGER_DOCKER_UPDATE_PRUNE=true
 # error info debug trace
 WUD_LOG_LEVEL=debug
 # text json