diff --git a/application/clicommands/ProcessCommand.php b/application/clicommands/ProcessCommand.php
index c343c38..4bf09ed 100644
--- a/application/clicommands/ProcessCommand.php
+++ b/application/clicommands/ProcessCommand.php
@@ -185,7 +185,7 @@ class ProcessCommand extends Command
         }
     }
 
-    protected function renderProblemTree($tree, $useColors = false, $depth = 0, BpNode $parent = null)
+    protected function renderProblemTree($tree, $useColors = false, $depth = 0, ?BpNode $parent = null)
     {
         $output = '';
 
diff --git a/application/controllers/ProcessController.php b/application/controllers/ProcessController.php
index 329b258..c12f024 100644
--- a/application/controllers/ProcessController.php
+++ b/application/controllers/ProcessController.php
@@ -264,7 +264,7 @@ class ProcessController extends Controller
         $bp->applySimulation($simulation);
     }
 
-    protected function loadActionForm(BpConfig $bp, Node $node = null)
+    protected function loadActionForm(BpConfig $bp, ?Node $node = null)
     {
         $action = $this->params->get('action');
         $form = null;
@@ -720,7 +720,7 @@ class ProcessController extends Controller
         return $tabs;
     }
 
-    protected function handleFormatRequest(BpConfig $bp, BpNode $node = null)
+    protected function handleFormatRequest(BpConfig $bp, ?BpNode $node = null)
     {
         $desiredContentType = $this->getRequest()->getHeader('Accept');
         if ($desiredContentType === 'application/json') {
diff --git a/application/forms/AddNodeForm.php b/application/forms/AddNodeForm.php
index 0e1aa0f..394a8d4 100644
--- a/application/forms/AddNodeForm.php
+++ b/application/forms/AddNodeForm.php
@@ -73,7 +73,7 @@ class AddNodeForm extends CompatForm
      *
      * @return $this
      */
-    public function setParentNode(BpNode $node = null): self
+    public function setParentNode(?BpNode $node = null): self
     {
         $this->parent = $node;
 
diff --git a/application/forms/DeleteNodeForm.php b/application/forms/DeleteNodeForm.php
index 7021506..2cb5e0e 100644
--- a/application/forms/DeleteNodeForm.php
+++ b/application/forms/DeleteNodeForm.php
@@ -84,7 +84,7 @@ class DeleteNodeForm extends BpConfigBaseForm
      * @param BpNode|null $node
      * @return $this
      */
-    public function setParentNode(BpNode $node = null)
+    public function setParentNode(?BpNode $node = null)
     {
         $this->parentNode = $node;
         return $this;
diff --git a/application/forms/EditNodeForm.php b/application/forms/EditNodeForm.php
index bd1592b..1ff938a 100644
--- a/application/forms/EditNodeForm.php
+++ b/application/forms/EditNodeForm.php
@@ -76,7 +76,7 @@ class EditNodeForm extends CompatForm
      *
      * @return $this
      */
-    public function setParentNode(BpNode $node = null): self
+    public function setParentNode(?BpNode $node = null): self
     {
         $this->parent = $node;
 
diff --git a/application/forms/MoveNodeForm.php b/application/forms/MoveNodeForm.php
index 0467e51..a56f15e 100644
--- a/application/forms/MoveNodeForm.php
+++ b/application/forms/MoveNodeForm.php
@@ -110,7 +110,7 @@ class MoveNodeForm extends BpConfigBaseForm
      * @param BpNode|null $node
      * @return $this
      */
-    public function setParentNode(BpNode $node = null)
+    public function setParentNode(?BpNode $node = null)
     {
         $this->parentNode = $node;
         return $this;
diff --git a/library/Businessprocess/Metadata.php b/library/Businessprocess/Metadata.php
index b640fb8..c3492c1 100644
--- a/library/Businessprocess/Metadata.php
+++ b/library/Businessprocess/Metadata.php
@@ -112,7 +112,7 @@ class Metadata
         return Auth::getInstance();
     }
 
-    public function canModify(Auth $auth = null)
+    public function canModify(?Auth $auth = null)
     {
         if ($auth === null) {
             if (Icinga::app()->isCli()) {
@@ -128,7 +128,7 @@ class Metadata
         );
     }
 
-    public function canRead(Auth $auth = null)
+    public function canRead(?Auth $auth = null)
     {
         if ($auth === null) {
             if (Icinga::app()->isCli()) {
diff --git a/library/Businessprocess/Modification/ProcessChanges.php b/library/Businessprocess/Modification/ProcessChanges.php
index 9257558..bec06b7 100644
--- a/library/Businessprocess/Modification/ProcessChanges.php
+++ b/library/Businessprocess/Modification/ProcessChanges.php
@@ -69,12 +69,12 @@ class ProcessChanges
     }
 
     /**
-     * @param Node $node
+     * @param ?Node $node
      * @param $properties
      *
      * @return $this
      */
-    public function addChildrenToNode($children, Node $node = null)
+    public function addChildrenToNode($children, ?Node $node = null)
     {
         $action = new NodeAddChildrenAction($node);
         $action->setChildren($children);
@@ -88,7 +88,7 @@ class ProcessChanges
      *
      * @return $this
      */
-    public function createNode($nodeName, $properties, Node $parent = null)
+    public function createNode($nodeName, $properties, ?Node $parent = null)
     {
         $action = new NodeCreateAction($nodeName);
         $action->setProperties($properties);
diff --git a/library/Businessprocess/Monitoring/DataView/HostStatus.php b/library/Businessprocess/Monitoring/DataView/HostStatus.php
index edc1814..3c22aa0 100644
--- a/library/Businessprocess/Monitoring/DataView/HostStatus.php
+++ b/library/Businessprocess/Monitoring/DataView/HostStatus.php
@@ -7,7 +7,7 @@ use Icinga\Module\Businessprocess\Monitoring\Backend\Ido\Query\HostStatusQuery;
 
 class HostStatus extends \Icinga\Module\Monitoring\DataView\Hoststatus
 {
-    public function __construct(ConnectionInterface $connection, array $columns = null)
+    public function __construct(ConnectionInterface $connection, ?array $columns = null)
     {
         parent::__construct($connection, $columns);
 
diff --git a/library/Businessprocess/Monitoring/DataView/ServiceStatus.php b/library/Businessprocess/Monitoring/DataView/ServiceStatus.php
index f3a9c3c..33a2221 100644
--- a/library/Businessprocess/Monitoring/DataView/ServiceStatus.php
+++ b/library/Businessprocess/Monitoring/DataView/ServiceStatus.php
@@ -7,7 +7,7 @@ use Icinga\Module\Businessprocess\Monitoring\Backend\Ido\Query\ServiceStatusQuer
 
 class ServiceStatus extends \Icinga\Module\Monitoring\DataView\Servicestatus
 {
-    public function __construct(ConnectionInterface $connection, array $columns = null)
+    public function __construct(ConnectionInterface $connection, ?array $columns = null)
     {
         parent::__construct($connection, $columns);
 
diff --git a/library/Businessprocess/Node.php b/library/Businessprocess/Node.php
index 1b899a7..da2d706 100644
--- a/library/Businessprocess/Node.php
+++ b/library/Businessprocess/Node.php
@@ -536,12 +536,12 @@ abstract class Node
     /**
      * Export the node to array
      *
-     * @param   array   $parent The node's parent. Used to construct the path to the node
+     * @param   ?array  $parent The node's parent. Used to construct the path to the node
      * @param   bool    $flat   If false, children will be added to the array key children, else the array will be flat
      *
      * @return  array
      */
-    public function toArray(array $parent = null, $flat = false)
+    public function toArray(?array $parent = null, $flat = false)
     {
         $data = [
             'name'  => $this->getAlias(),
diff --git a/library/Businessprocess/Renderer/Renderer.php b/library/Businessprocess/Renderer/Renderer.php
index d7551ab..13c300c 100644
--- a/library/Businessprocess/Renderer/Renderer.php
+++ b/library/Businessprocess/Renderer/Renderer.php
@@ -47,7 +47,7 @@ abstract class Renderer extends HtmlDocument
      * @param BpConfig $config
      * @param BpNode|null $parent
      */
-    public function __construct(BpConfig $config, BpNode $parent = null)
+    public function __construct(BpConfig $config, ?BpNode $parent = null)
     {
         $this->config = $config;
         $this->parent = $parent;
diff --git a/library/Businessprocess/Renderer/TreeRenderer.php b/library/Businessprocess/Renderer/TreeRenderer.php
index ed25e00..1b4aae7 100644
--- a/library/Businessprocess/Renderer/TreeRenderer.php
+++ b/library/Businessprocess/Renderer/TreeRenderer.php
@@ -112,11 +112,11 @@ class TreeRenderer extends Renderer
 
     /**
      * @param Node $node
-     * @param array $path
-     * @param BpNode $parent
+     * @param ?array $path
+     * @param ?BpNode $parent
      * @return BaseHtmlElement[]
      */
-    public function getNodeIcons(Node $node, array $path = null, BpNode $parent = null)
+    public function getNodeIcons(Node $node, ?array $path = null, ?BpNode $parent = null)
     {
         $icons = [];
         if (empty($path) && $node instanceof BpNode) {
diff --git a/library/Businessprocess/Web/Form/FormLoader.php b/library/Businessprocess/Web/Form/FormLoader.php
index 0cc5389..86e90cf 100644
--- a/library/Businessprocess/Web/Form/FormLoader.php
+++ b/library/Businessprocess/Web/Form/FormLoader.php
@@ -8,7 +8,7 @@ use Icinga\Exception\ProgrammingError;
 
 class FormLoader
 {
-    public static function load($name, Module $module = null)
+    public static function load($name, ?Module $module = null)
     {
         if ($module === null) {
             $basedir = Icinga::app()->getApplicationDir('forms');
diff --git a/library/Businessprocess/Web/Form/QuickBaseForm.php b/library/Businessprocess/Web/Form/QuickBaseForm.php
index 6a60d69..b5d8961 100644
--- a/library/Businessprocess/Web/Form/QuickBaseForm.php
+++ b/library/Businessprocess/Web/Form/QuickBaseForm.php
@@ -140,7 +140,7 @@ abstract class QuickBaseForm extends Zend_Form implements ValidHtml
         return $this;
     }
 
-    protected function loadForm($name, Module $module = null)
+    protected function loadForm($name, ?Module $module = null)
     {
         if ($module === null) {
             $module = $this->icingaModule;
diff --git a/library/Businessprocess/Web/Form/QuickForm.php b/library/Businessprocess/Web/Form/QuickForm.php
index 5326da3..2420678 100644
--- a/library/Businessprocess/Web/Form/QuickForm.php
+++ b/library/Businessprocess/Web/Form/QuickForm.php
@@ -325,7 +325,7 @@ abstract class QuickForm extends QuickBaseForm
         return $this;
     }
 
-    public function handleRequest(Request $request = null)
+    public function handleRequest(?Request $request = null)
     {
         if ($request === null) {
             $request = $this->getRequest();