diff --git a/src/Entity/Base/AbstractDBElement.php b/src/Entity/Base/AbstractDBElement.php
index a186b489..3a97473c 100644
--- a/src/Entity/Base/AbstractDBElement.php
+++ b/src/Entity/Base/AbstractDBElement.php
@@ -87,7 +87,7 @@ abstract class AbstractDBElement implements \JsonSerializable
         return $this->id;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return ['@id' => $this->getID()];
     }
diff --git a/src/Helpers/Trees/StructuralDBElementIterator.php b/src/Helpers/Trees/StructuralDBElementIterator.php
index 488aec0c..1bbbd6ef 100644
--- a/src/Helpers/Trees/StructuralDBElementIterator.php
+++ b/src/Helpers/Trees/StructuralDBElementIterator.php
@@ -62,7 +62,7 @@ final class StructuralDBElementIterator extends ArrayIterator implements Recursi
         return !empty($element->getSubelements());
     }
 
-    public function getChildren()
+    public function getChildren(): StructuralDBElementIterator
     {
         /** @var AbstractStructuralDBElement $element */
         $element = $this->current();
diff --git a/src/Helpers/Trees/TreeViewNode.php b/src/Helpers/Trees/TreeViewNode.php
index 889eed97..a905cd93 100644
--- a/src/Helpers/Trees/TreeViewNode.php
+++ b/src/Helpers/Trees/TreeViewNode.php
@@ -233,7 +233,7 @@ final class TreeViewNode implements JsonSerializable
         return $this;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         $ret = [
             'text' => $this->text,
diff --git a/src/Helpers/Trees/TreeViewNodeIterator.php b/src/Helpers/Trees/TreeViewNodeIterator.php
index e57f8943..46fd8343 100644
--- a/src/Helpers/Trees/TreeViewNodeIterator.php
+++ b/src/Helpers/Trees/TreeViewNodeIterator.php
@@ -63,7 +63,7 @@ final class TreeViewNodeIterator extends ArrayIterator implements RecursiveItera
         return !empty($element->getNodes());
     }
 
-    public function getChildren()
+    public function getChildren(): TreeViewNodeIterator
     {
         /** @var TreeViewNode $element */
         $element = $this->current();