diff --git a/tests/Phug/AbstractLexerTest.php b/tests/Phug/AbstractLexerTest.php
index eba9ccfc..91ff686c 100644
--- a/tests/Phug/AbstractLexerTest.php
+++ b/tests/Phug/AbstractLexerTest.php
@@ -49,7 +49,7 @@ protected function filterTokenClass($className)
         }
     }
 
-    protected function assertTokens($expression, array $classNames, Lexer $lexer = null, &$tokens = [])
+    protected function assertTokens($expression, array $classNames, ?Lexer $lexer = null, &$tokens = [])
     {
         $lexer = $lexer ?: $this->lexer;
         $tokens = iterator_to_array($lexer->lex($expression));
diff --git a/tests/Phug/AbstractParserTest.php b/tests/Phug/AbstractParserTest.php
index 8774e528..5816c775 100644
--- a/tests/Phug/AbstractParserTest.php
+++ b/tests/Phug/AbstractParserTest.php
@@ -17,7 +17,7 @@ protected function prepareTest()
         $this->parser = new Parser();
     }
 
-    protected function assertNodes($expression, $expected, Parser $parser = null)
+    protected function assertNodes($expression, $expected, ?Parser $parser = null)
     {
         if (is_array($expected)) {
             $expected = implode("\n", $expected);
diff --git a/tests/Phug/Compiler/NodeCompiler/BlockNodeCompilerTest/TestBlockNodeCompiler.php b/tests/Phug/Compiler/NodeCompiler/BlockNodeCompilerTest/TestBlockNodeCompiler.php
index 015e1b7b..b13fa5a9 100644
--- a/tests/Phug/Compiler/NodeCompiler/BlockNodeCompilerTest/TestBlockNodeCompiler.php
+++ b/tests/Phug/Compiler/NodeCompiler/BlockNodeCompilerTest/TestBlockNodeCompiler.php
@@ -9,7 +9,7 @@
 
 class TestBlockNodeCompiler extends BlockNodeCompiler
 {
-    public function compileNode(NodeInterface $node, ElementInterface $parent = null)
+    public function compileNode(NodeInterface $node, ?ElementInterface $parent = null)
     {
         $blocks = &$this->getCompiler()->getBlocksByName('foo');
         $blocks[] = 'bar';
diff --git a/tests/Phug/TestCompiler.php b/tests/Phug/TestCompiler.php
index 63f848b7..b90ab72e 100644
--- a/tests/Phug/TestCompiler.php
+++ b/tests/Phug/TestCompiler.php
@@ -8,7 +8,7 @@
 
 class TestCompiler extends Compiler
 {
-    public function compileNode(NodeInterface $node, ElementInterface $parent = null)
+    public function compileNode(NodeInterface $node, ?ElementInterface $parent = null)
     {
         return 'foo';
     }
diff --git a/tests/Phug/TestState.php b/tests/Phug/TestState.php
index 6f356ee8..7b9d5bf5 100644
--- a/tests/Phug/TestState.php
+++ b/tests/Phug/TestState.php
@@ -9,7 +9,7 @@ class TestState extends State
 {
     private static $lastOptions;
 
-    public function __construct(Parser $parser, \Generator $tokens, array $options = null)
+    public function __construct(Parser $parser, \Generator $tokens, ?array $options = null)
     {
         parent::__construct($parser, $tokens, $options);
 
diff --git a/tests/Phug/Utils/MutedExceptionCompiler.php b/tests/Phug/Utils/MutedExceptionCompiler.php
index 5efa7915..b8f17e3c 100644
--- a/tests/Phug/Utils/MutedExceptionCompiler.php
+++ b/tests/Phug/Utils/MutedExceptionCompiler.php
@@ -15,7 +15,7 @@ public function throwException($message, $node = null, $code = 0, $previous = nu
         // removed
     }
 
-    public function compileNode(NodeInterface $node, ElementInterface $parent = null)
+    public function compileNode(NodeInterface $node, ?ElementInterface $parent = null)
     {
         return $this->forcedReturn ?: parent::compileNode($node, $parent);
     }
diff --git a/tests/Phug/Utils/TwigFormat.php b/tests/Phug/Utils/TwigFormat.php
index 0daf3e9b..fadefa0e 100644
--- a/tests/Phug/Utils/TwigFormat.php
+++ b/tests/Phug/Utils/TwigFormat.php
@@ -8,7 +8,7 @@
 
 class TwigFormat extends XhtmlFormat
 {
-    public function __construct(Formatter $formatter = null)
+    public function __construct(?Formatter $formatter = null)
     {
         parent::__construct($formatter);