diff --git a/update-berths.pl b/update-berths.pl
index 952a863..a5e2946 100755
--- a/update-berths.pl
+++ b/update-berths.pl
@@ -85,7 +85,7 @@ sub set_berth($$$) {
 
 sub clear_berth($$) {
     my ($area, $berth) = @_;
-    publish(sprintf("berths/%s/%s", $area||"-", $berth || "-"), "-");
+    publish(sprintf("berths/%s/%s", $area||"-", $berth || "-"), undef);
 }
 
 sub publish($$) {