diff --git a/lib/WellKnown/WebfingerHandler.php b/lib/WellKnown/WebfingerHandler.php
index fee8c4b28..ff3559eaf 100644
--- a/lib/WellKnown/WebfingerHandler.php
+++ b/lib/WellKnown/WebfingerHandler.php
@@ -97,6 +97,12 @@ public function handle(
 	 */
 	public function handleWebfinger(IRequestContext $context, ?IResponse $previousResponse): ?IResponse {
 		$subject = $this->getSubjectFromRequest($context->getHttpRequest());
+		
+		// the `resource` parameter is required
+		if ($subject === '') {
+			return new JrdResponse('', Http::STATUS_BAD_REQUEST);
+		}
+		
 		if (str_starts_with($subject, 'acct:')) {
 			$subject = substr($subject, 5);
 		}
@@ -216,8 +222,11 @@ private function getSubjectFromRequest(IRequest $request): string {
 
 		// work around to extract resource:
 		// on some setup (i.e. tests) the data are not available from IRequest
-		parse_str(parse_url($request->getRequestUri(), PHP_URL_QUERY), $query);
-
+		$requestUri = $request->getRequestUri();
+		if ($requestUri !== '') {
+			parse_str(parse_url($requestUri, PHP_URL_QUERY) ?? '', $query);
+		}
+		
 		return $query['resource'] ?? '';
 	}
 }