diff --git a/Models/NullCategory.php b/Models/NullCategory.php
index 8215089..306d2ba 100755
--- a/Models/NullCategory.php
+++ b/Models/NullCategory.php
@@ -35,4 +35,12 @@ final class NullCategory extends Category
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullCause.php b/Models/NullCause.php
index e992184..b0655e6 100755
--- a/Models/NullCause.php
+++ b/Models/NullCause.php
@@ -35,4 +35,12 @@ final class NullCause extends Cause
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullDepartment.php b/Models/NullDepartment.php
index 3de9bbf..bd4ffda 100755
--- a/Models/NullDepartment.php
+++ b/Models/NullDepartment.php
@@ -35,4 +35,12 @@ final class NullDepartment extends Department
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProcess.php b/Models/NullProcess.php
index 91192e1..5e5e203 100755
--- a/Models/NullProcess.php
+++ b/Models/NullProcess.php
@@ -35,4 +35,12 @@ final class NullProcess extends Process
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProject.php b/Models/NullProject.php
index c47632e..25de963 100755
--- a/Models/NullProject.php
+++ b/Models/NullProject.php
@@ -35,4 +35,12 @@ final class NullProject extends Project
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullRisk.php b/Models/NullRisk.php
index 26c6ab3..6a8ad3e 100755
--- a/Models/NullRisk.php
+++ b/Models/NullRisk.php
@@ -35,4 +35,12 @@ final class NullRisk extends Risk
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullRiskObject.php b/Models/NullRiskObject.php
index e0117f2..87d742a 100755
--- a/Models/NullRiskObject.php
+++ b/Models/NullRiskObject.php
@@ -35,4 +35,12 @@ final class NullRiskObject extends RiskObject
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullSolution.php b/Models/NullSolution.php
index 023c30a..6703366 100755
--- a/Models/NullSolution.php
+++ b/Models/NullSolution.php
@@ -35,4 +35,12 @@ final class NullSolution extends Solution
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }