diff --git a/Parsedown.php b/Parsedown.php
index ae0cbde..fb5239d 100644
--- a/Parsedown.php
+++ b/Parsedown.php
@@ -571,7 +571,7 @@ class Parsedown
     #
     # List
 
-    protected function blockList($Line, array $CurrentBlock = null)
+    protected function blockList($Line, ?array $CurrentBlock = null)
     {
         list($name, $pattern) = $Line['text'][0] <= '-' ? array('ul', '[*+-]') : array('ol', '[0-9]{1,9}+[.\)]');
 
@@ -808,7 +808,7 @@ class Parsedown
     #
     # Setext
 
-    protected function blockSetextHeader($Line, array $Block = null)
+    protected function blockSetextHeader($Line, ?array $Block = null)
     {
         if ( ! isset($Block) or $Block['type'] !== 'Paragraph' or isset($Block['interrupted']))
         {
@@ -894,7 +894,7 @@ class Parsedown
     #
     # Table
 
-    protected function blockTable($Line, array $Block = null)
+    protected function blockTable($Line, ?array $Block = null)
     {
         if ( ! isset($Block) or $Block['type'] !== 'Paragraph' or isset($Block['interrupted']))
         {