diff --git a/filter.sh b/filter.sh
index f17cddf..c8f585b 100755
--- a/filter.sh
+++ b/filter.sh
@@ -242,6 +242,6 @@ case "$1" in
             filter_entries
             mark_as_read
             # warte zeit x
-            sleep $MF_SLEEP
+            sleep "$MF_SLEEP"
         done
 esac