diff --git a/src/Common.php b/src/Common.php
index 20bf43e..93a3f69 100644
--- a/src/Common.php
+++ b/src/Common.php
@@ -48,7 +48,7 @@ function inflectRecord($record): Maybe
  *
  * @return mixed|null
  */
-function memoize(callable $callback = null, array $arguments = [], $key = null)
+function memoize(?callable $callback = null, array $arguments = [], $key = null)
 {
     static $storage = [];
 
diff --git a/src/Objects/Exceptions/Exceptions.php b/src/Objects/Exceptions/Exceptions.php
index 0b2a96b..b85908b 100644
--- a/src/Objects/Exceptions/Exceptions.php
+++ b/src/Objects/Exceptions/Exceptions.php
@@ -12,7 +12,7 @@ final class Exceptions
     /**
      * @param mixed $record
      */
-    public static function forEncode($record, \AvroSchema $schema, \Exception $previous = null): AvroEncodingException
+    public static function forEncode($record, \AvroSchema $schema, ?\Exception $previous = null): AvroEncodingException
     {
         $exportedRecord = \var_export($record, true);
 
@@ -29,7 +29,7 @@ public static function forEncode($record, \AvroSchema $schema, \Exception $previ
         return new AvroEncodingException($message, self::ERROR_ENCODING, $previous);
     }
 
-    public static function forDecode(string $binaryMessage, \Exception $previous = null): AvroDecodingException
+    public static function forDecode(string $binaryMessage, ?\Exception $previous = null): AvroDecodingException
     {
         $convertedMessage = \bin2hex($binaryMessage);
         $message = <<<MESSAGE
diff --git a/src/Objects/RecordSerializer.php b/src/Objects/RecordSerializer.php
index 749c42a..330fc38 100644
--- a/src/Objects/RecordSerializer.php
+++ b/src/Objects/RecordSerializer.php
@@ -131,7 +131,7 @@ public function encodeRecord(string $subject, AvroSchema $schema, $record): stri
      * @throws \Exception
      * @throws \FlixTech\SchemaRegistryApi\Exception\SchemaRegistryException
      */
-    public function decodeMessage(string $binaryMessage, AvroSchema $readersSchema = null)
+    public function decodeMessage(string $binaryMessage, ?AvroSchema $readersSchema = null)
     {
         $decoded = decode($binaryMessage);
         $schemaId = valueOf($decoded->bind($this->schemaIdGetter));
diff --git a/src/Objects/Schema/Generation/Type.php b/src/Objects/Schema/Generation/Type.php
index 75a6035..08f3f19 100644
--- a/src/Objects/Schema/Generation/Type.php
+++ b/src/Objects/Schema/Generation/Type.php
@@ -16,7 +16,7 @@ class Type
      */
     private $attributes;
 
-    public function __construct(string $typeName, SchemaAttributes $attributes = null)
+    public function __construct(string $typeName, ?SchemaAttributes $attributes = null)
     {
         $this->typeName = $typeName;
         $this->attributes = $attributes ?? new SchemaAttributes();
diff --git a/test/Objects/SchemaResolvers/DefinitionInterfaceResolverTest.php b/test/Objects/SchemaResolvers/DefinitionInterfaceResolverTest.php
index 56c56b9..5d40c71 100644
--- a/test/Objects/SchemaResolvers/DefinitionInterfaceResolverTest.php
+++ b/test/Objects/SchemaResolvers/DefinitionInterfaceResolverTest.php
@@ -65,7 +65,7 @@ public function it_should_fail_for_records_not_implementing_the_interface_for_ke
         $definitionInterfaceResolver->keySchemaFor([]);
     }
 
-    private function createAnonymousDefinitionInterface(string $valueSchema, string $keySchema = null): HasSchemaDefinitionInterface
+    private function createAnonymousDefinitionInterface(string $valueSchema, ?string $keySchema = null): HasSchemaDefinitionInterface
     {
         $class = new class() implements HasSchemaDefinitionInterface {
             /**