diff --git a/seafile_sync/Dockerfile b/seafile_sync/Dockerfile
index c772be4..7f8b6d6 100644
--- a/seafile_sync/Dockerfile
+++ b/seafile_sync/Dockerfile
@@ -1,4 +1,5 @@
 # https://gitlab.com/flwgns-docker/seafile-client/
 ARG \
-    SEAF_SERVER_URL
+    SEAF_SERVER_URL==${SEAF_SERVER_URL}
+
 FROM flowgunso/seafile-client:latest