diff --git a/src/Iterator/PostOrderTraversal.php b/src/Iterator/PostOrderTraversal.php
index 75868f7..f527ecc 100644
--- a/src/Iterator/PostOrderTraversal.php
+++ b/src/Iterator/PostOrderTraversal.php
@@ -37,7 +37,7 @@ public function getIterator(): Generator
         );
     }
 
-    private function generate(TreeNodeContract $node, array $vector, int $nodeSeq, Counter $counter)
+    private function generate(TreeNodeContract $node, array $vector, int $nodeSeq, Counter $counter): Generator
     {
         // $seq is the child sequence number, within the given parent node.
         $seq = 0;
diff --git a/src/Iterator/PreOrderTraversal.php b/src/Iterator/PreOrderTraversal.php
index dfcbfdc..59fdfaf 100644
--- a/src/Iterator/PreOrderTraversal.php
+++ b/src/Iterator/PreOrderTraversal.php
@@ -37,7 +37,7 @@ public function getIterator(): Generator
         );
     }
 
-    private function generate(TreeNodeContract $node, array $vector, int $nodeSeq, Counter $counter)
+    private function generate(TreeNodeContract $node, array $vector, int $nodeSeq, Counter $counter): Generator
     {
         // The yielded key is calculated by the key function.
         // By default, it returns an incrementing sequence to prevent issues with `iterator_to_array` casts.