diff --git a/src/Sql/AbstractInsertStatement.php b/src/Sql/AbstractInsertStatement.php
index 7d334ff..966e29b 100644
--- a/src/Sql/AbstractInsertStatement.php
+++ b/src/Sql/AbstractInsertStatement.php
@@ -23,7 +23,7 @@ abstract class AbstractInsertStatement extends AbstractStatement implements Comm
      * @param string|null $sequence Name of the sequence object from which the ID should be returned.
      * @return mixed Returns the ID of the last inserted row or sequence value.
      */
-    public function exec(string $sequence = null): mixed
+    public function exec(?string $sequence = null): mixed
     {
         return $this->db()->insert($this->toSql(), $this->getParams(), $sequence);
     }
diff --git a/src/Sql/AbstractStatement.php b/src/Sql/AbstractStatement.php
index a1c1aae..15d24c1 100644
--- a/src/Sql/AbstractStatement.php
+++ b/src/Sql/AbstractStatement.php
@@ -19,7 +19,7 @@ abstract class AbstractStatement implements Statement
      */
     protected bool $built = false;
 
-    final public function __construct(StatementExecutor $db = null)
+    final public function __construct(?StatementExecutor $db = null)
     {
         $this->db = $db;
     }
diff --git a/src/StatementExecutor.php b/src/StatementExecutor.php
index 4b1f0d4..874553a 100644
--- a/src/StatementExecutor.php
+++ b/src/StatementExecutor.php
@@ -23,7 +23,7 @@ public function execute(string $sql, array $params): int;
      *
      * @return mixed Returns the ID of the last inserted row or sequence value.
      */
-    public function insert(string $sql, array $params, string $sequence = null): mixed;
+    public function insert(string $sql, array $params, ?string $sequence = null): mixed;
 
     /**
      * Executes the SQL statement and returns all rows.