diff --git a/openstreetmap-tiles-update-expire.sh b/openstreetmap-tiles-update-expire.sh
index e4a3fe7..e64c150 100755
--- a/openstreetmap-tiles-update-expire.sh
+++ b/openstreetmap-tiles-update-expire.sh
@@ -56,7 +56,7 @@ RUNLOG=$LOG_DIR/run.log
 # Here we expire (but don't immediately rerender) tiles between zoom levels
 # 13 and 18 and delete between 19 and 20.
 #------------------------------------------------------------------------------
-EXPIRY_MINZOOM=${EXPIRY_MINZOOM:="13}
+EXPIRY_MINZOOM=${EXPIRY_MINZOOM:="13"}
 EXPIRY_TOUCHFROM=${EXPIRY_TOUCHFROM:="13"}
 EXPIRY_DELETEFROM=${EXPIRY_DELETEFROM:="19"}
 EXPIRY_MAXZOOM=${EXPIRY_MAXZOOM:="20"}