diff --git a/src/CardDavClient.php b/src/CardDavClient.php
index 0aee708..7d3fbaf 100644
--- a/src/CardDavClient.php
+++ b/src/CardDavClient.php
@@ -582,8 +582,8 @@ public static function compareUrlPaths(string $url1, string $url2): bool
     {
         $comp1 = \Sabre\Uri\parse($url1);
         $comp2 = \Sabre\Uri\parse($url2);
-        $p1 = trim(rtrim($comp1["path"], "/"), "/");
-        $p2 = trim(rtrim($comp2["path"], "/"), "/");
+        $p1 = trim(rtrim($comp1["path"] ?? '', "/"), "/");
+        $p2 = trim(rtrim($comp2["path"] ?? '', "/"), "/");
         return $p1 === $p2;
     }
 
diff --git a/src/Services/Sync.php b/src/Services/Sync.php
index bb2a87b..a420ec6 100644
--- a/src/Services/Sync.php
+++ b/src/Services/Sync.php
@@ -287,7 +287,7 @@ private function determineChangesViaETags(
                         || ($etag !== $localCacheState[$uri])
                     ) {
                         $changes[] = [
-                            'uri' => $uri,
+                            'uri' => $uri ?? '',
                             'etag' => $etag
                         ];
                     }