diff --git a/lib/ClustersBackend/AlbumsBackend.php b/lib/ClustersBackend/AlbumsBackend.php
index dafa813d..83c498cd 100644
--- a/lib/ClustersBackend/AlbumsBackend.php
+++ b/lib/ClustersBackend/AlbumsBackend.php
@@ -112,7 +112,7 @@ class AlbumsBackend extends Backend
         return array_values($list);
     }
 
-    public static function getClusterId(array $cluster)
+    public static function getClusterId(array $cluster): int|string
     {
         return $cluster['cluster_id'];
     }
diff --git a/lib/ClustersBackend/Backend.php b/lib/ClustersBackend/Backend.php
index 63ae203f..faa9a9c4 100644
--- a/lib/ClustersBackend/Backend.php
+++ b/lib/ClustersBackend/Backend.php
@@ -69,10 +69,8 @@ abstract class Backend
 
     /**
      * Get a cluster ID for the given cluster.
-     *
-     * @return int|string
      */
-    abstract public static function getClusterId(array $cluster);
+    abstract public static function getClusterId(array $cluster): int|string;
 
     /**
      * Get a list of photos with any extra parameters for the given cluster
diff --git a/lib/ClustersBackend/FaceRecognitionBackend.php b/lib/ClustersBackend/FaceRecognitionBackend.php
index 5ba0fade..92c6c8b4 100644
--- a/lib/ClustersBackend/FaceRecognitionBackend.php
+++ b/lib/ClustersBackend/FaceRecognitionBackend.php
@@ -132,7 +132,7 @@ class FaceRecognitionBackend extends Backend
         return $faces;
     }
 
-    public static function getClusterId(array $cluster)
+    public static function getClusterId(array $cluster): int|string
     {
         return $cluster['id'];
     }
diff --git a/lib/ClustersBackend/PlacesBackend.php b/lib/ClustersBackend/PlacesBackend.php
index 68f2a808..bdef23dd 100644
--- a/lib/ClustersBackend/PlacesBackend.php
+++ b/lib/ClustersBackend/PlacesBackend.php
@@ -168,7 +168,7 @@ class PlacesBackend extends Backend
         return $places;
     }
 
-    public static function getClusterId(array $cluster)
+    public static function getClusterId(array $cluster): int|string
     {
         return $cluster['osm_id'];
     }
diff --git a/lib/ClustersBackend/RecognizeBackend.php b/lib/ClustersBackend/RecognizeBackend.php
index 615cabae..29152f40 100644
--- a/lib/ClustersBackend/RecognizeBackend.php
+++ b/lib/ClustersBackend/RecognizeBackend.php
@@ -185,7 +185,7 @@ class RecognizeBackend extends Backend
         return $faces;
     }
 
-    public static function getClusterId(array $cluster)
+    public static function getClusterId(array $cluster): int|string
     {
         return $cluster['id'];
     }
diff --git a/lib/ClustersBackend/TagsBackend.php b/lib/ClustersBackend/TagsBackend.php
index 0410b42e..9d01d56b 100644
--- a/lib/ClustersBackend/TagsBackend.php
+++ b/lib/ClustersBackend/TagsBackend.php
@@ -106,7 +106,7 @@ class TagsBackend extends Backend
         return $tags;
     }
 
-    public static function getClusterId(array $cluster)
+    public static function getClusterId(array $cluster): int|string
     {
         return $cluster['name'];
     }