diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
index 00c8564..86be36d 100644
--- a/.github/workflows/run-tests.yml
+++ b/.github/workflows/run-tests.yml
@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: ["7.4", "8.0", "8.1", "8.2", "8.3"]
+        php: ["7.4", "8.0", "8.1", "8.2", "8.3", "8.4"]
 
     name: php-${{ matrix.php }}
 
diff --git a/src/Chess.php b/src/Chess.php
index 5d9df43..3364ac4 100644
--- a/src/Chess.php
+++ b/src/Chess.php
@@ -393,7 +393,7 @@ public function undo(): ?Move
     /**
      * @return array<int, Move>
      */
-    protected function generateMoves(int $square = null, bool $legal = true): array
+    protected function generateMoves(?int $square = null, bool $legal = true): array
     {
         $cacheKey = $this->boardHash.\json_encode($square.($legal ? '1' : '0'), JSON_THROW_ON_ERROR);
 
@@ -593,7 +593,7 @@ public function move($sanOrArray): ?Move
      *
      * @return array<int, Move>
      */
-    public function moves(int $square = null): array
+    public function moves(?int $square = null): array
     {
         $moves = [];
         foreach ($this->generateMoves($square) as $key => $move) {
diff --git a/src/History.php b/src/History.php
index 42e8ec0..c80ed3a 100644
--- a/src/History.php
+++ b/src/History.php
@@ -10,7 +10,7 @@ final class History
     private array $entries;
 
     /** @param array<int, Entry>|null $entries */
-    public function __construct(array $entries = null)
+    public function __construct(?array $entries = null)
     {
         $this->entries = $entries ?? [];
     }
diff --git a/src/Move.php b/src/Move.php
index 2b0f93d..759d6c8 100644
--- a/src/Move.php
+++ b/src/Move.php
@@ -34,7 +34,7 @@ final class Move implements \JsonSerializable
 
     public ?string $promotion;
 
-    public ?string $san;
+    public ?string $san = null;
 
     public function __construct(string $turn, int $flags, Piece $piece, int $from, int $to, ?string $captured = null, ?string $promotion = null)
     {
diff --git a/src/Piece.php b/src/Piece.php
index 53a2938..866e0fa 100644
--- a/src/Piece.php
+++ b/src/Piece.php
@@ -48,7 +48,7 @@ final class Piece implements \JsonSerializable
     private string $color;
 
     /** @var array<string> */
-    private static $types = [
+    private static array $types = [
         self::PAWN,
         self::KNIGHT,
         self::BISHOP,
@@ -58,10 +58,10 @@ final class Piece implements \JsonSerializable
     ];
 
     /** @var array<string> */
-    private static $colors = [self::BLACK, self::WHITE];
+    private static array $colors = [self::BLACK, self::WHITE];
 
     /** @var array<string, string[]> */
-    private static $pieces = [
+    private static array $pieces = [
         self::BLACK => [
             self::PAWN => '♟',
             self::KNIGHT => '♞',
diff --git a/tests/ChessPublicator.php b/tests/ChessPublicator.php
index 4b308fc..cc8155b 100644
--- a/tests/ChessPublicator.php
+++ b/tests/ChessPublicator.php
@@ -30,7 +30,7 @@ public function attackedPublic(string $color, int $square): bool
     /**
      * @return array<int, Move>
      */
-    public function generateMovesPublic(int $square = null, bool $legal = true): array
+    public function generateMovesPublic(?int $square = null, bool $legal = true): array
     {
         return $this->generateMoves($square, $legal);
     }
diff --git a/tests/MoveTest.php b/tests/MoveTest.php
index cd142ce..dd04364 100644
--- a/tests/MoveTest.php
+++ b/tests/MoveTest.php
@@ -488,7 +488,7 @@ public static function testSANMoveFromRealGame(string $match, string $finalFen):
     /**
      * @return array<string, array<int, string>>
      */
-    public function gameProvider(): array
+    public static function gameProvider(): array
     {
         $match1 = 'e4 e5 Nf3 Nc6 d4 exd4 Nxd4 Nf6 Nc3 Bb4 Nxc6 bxc6 Qd4 Qe7 f3 d5 Bg5 O-O O-O-O Bc5 Bxf6 gxf6 Qa4 '.
             'Be3+ Kb1 d4 Ne2 c5 Nc1 Be6 Bc4 Rfb8 Nd3 Rb6';
diff --git a/tests/PerftTest.php b/tests/PerftTest.php
index 661831c..701dd51 100644
--- a/tests/PerftTest.php
+++ b/tests/PerftTest.php
@@ -23,7 +23,7 @@ public static function testPerft(string $fen, int $expectedDeep1, int $expectedD
     /**
      * @return array<string, array<int, int|string>>
      */
-    public function provider(): array
+    public static function provider(): array
     {
         return [
             'initial position' => ['rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1', 20, 400],