diff --git a/src/network/upnp/UPnP.php b/src/network/upnp/UPnP.php
index 2d48a2db83d..bd8e8376f3c 100644
--- a/src/network/upnp/UPnP.php
+++ b/src/network/upnp/UPnP.php
@@ -215,6 +215,7 @@ public static function portForward(string $serviceURL, string $internalIP, int $
 			'SOAPAction: "urn:schemas-upnp-org:service:WANIPConnection:1#AddPortMapping"'
 		];
 
+		$err = "";
 		if(Internet::postURL($serviceURL, $contents, 3, $headers, $err) === null){
 			throw new UPnPException("Failed to portforward using UPnP: " . $err);
 		}