diff --git a/Models/Department.php b/Models/Department.php
index 39c3d20..fa45dc7 100644
--- a/Models/Department.php
+++ b/Models/Department.php
@@ -45,7 +45,7 @@ class Department
 
     public function getParent()
     {
-        return $this->parent;
+        return $this->parent ?? new NullDepartment();
     }
 
     public function setParent(int $parent)
@@ -65,7 +65,7 @@ class Department
 
     public function getUnit()
     {
-        return $this->unit;
+        return $this->unit ?? new NullUnit();
     }
 
     public function setUnit($unit)
diff --git a/Models/Position.php b/Models/Position.php
index ffc0145..5883ac6 100644
--- a/Models/Position.php
+++ b/Models/Position.php
@@ -47,7 +47,7 @@ class Position implements ArrayableInterface, \JsonSerializable
 
     public function getParent()
     {
-        return $this->parent;
+        return $this->parent ?? new NullPosition();
     }
 
     public function setParent(int $parent)
@@ -57,7 +57,7 @@ class Position implements ArrayableInterface, \JsonSerializable
     
     public function getDepartment()
     {
-        return $this->department;
+        return $this->department ?? new NullDepartment();
     }
 
     public function setDepartment($department)
diff --git a/Models/Unit.php b/Models/Unit.php
index c6c63ab..8645416 100644
--- a/Models/Unit.php
+++ b/Models/Unit.php
@@ -45,7 +45,7 @@ class Unit implements ArrayableInterface, \JsonSerializable
 
     public function getParent()
     {
-        return $this->parent;
+        return $this->parent ?? new NullUnit();
     }
 
     public function setParent(int $parent)