diff --git a/src/Dev/Benchmark.php b/src/Dev/Benchmark.php
index a922954d..9bf00836 100644
--- a/src/Dev/Benchmark.php
+++ b/src/Dev/Benchmark.php
@@ -29,7 +29,7 @@ public static function start(string $id): void
      * @return string|null
      * @throws \Exception
      */
-    public static function end(string $id, string $message = null, bool $return = true): ?string
+    public static function end(string $id, ?string $message = null, bool $return = true): ?string
     {
         $benchmark = Benchmark::$benchmarks[$id] ?? null;
         if (!$benchmark) {
diff --git a/src/Dev/SchemaBuild.php b/src/Dev/SchemaBuild.php
index e12cee87..f3786e80 100644
--- a/src/Dev/SchemaBuild.php
+++ b/src/Dev/SchemaBuild.php
@@ -61,7 +61,7 @@ protected function getHeading(): string
      * @throws SchemaNotFoundException
      * @throws SchemaBuilderException
      */
-    public function buildSchema(string $key = null, bool $clear = true): void
+    public function buildSchema(?string $key = null, bool $clear = true): void
     {
         /** @var LoggerInterface $logger */
         $logger = Injector::inst()->get(LoggerInterface::class . '.graphql-build');
diff --git a/src/Schema/DataObject/CreateCreator.php b/src/Schema/DataObject/CreateCreator.php
index d3fa8802..ab3e8655 100644
--- a/src/Schema/DataObject/CreateCreator.php
+++ b/src/Schema/DataObject/CreateCreator.php
@@ -79,7 +79,7 @@ public function createOperation(
     public static function resolve(array $resolverContext = []): Closure
     {
         $dataClass = $resolverContext['dataClass'] ?? null;
-        return function ($obj, $args = [], $context = [], ResolveInfo $info = null) use ($dataClass) {
+        return function ($obj, $args = [], $context = [], ?ResolveInfo $info = null) use ($dataClass) {
             if (!$dataClass) {
                 return null;
             }
diff --git a/src/Schema/Services/SchemaTranscriber.php b/src/Schema/Services/SchemaTranscriber.php
index 16632152..0b1409d0 100644
--- a/src/Schema/Services/SchemaTranscriber.php
+++ b/src/Schema/Services/SchemaTranscriber.php
@@ -38,7 +38,7 @@ class SchemaTranscriber
      * @param string $rootDir Storage path for the generated file.
      *                        Caution: This location may be used by frontend assets relying on GraphQL, e.g. silverstripe/assets.
      */
-    public function __construct(GraphQLSchema $schema, string $name, string $rootDir = null)
+    public function __construct(GraphQLSchema $schema, string $name, ?string $rootDir = null)
     {
         $this->fs = new Filesystem();
         $this->schema = $schema;
diff --git a/tests/Schema/SchemaTest.php b/tests/Schema/SchemaTest.php
index 8ca2d078..28673276 100644
--- a/tests/Schema/SchemaTest.php
+++ b/tests/Schema/SchemaTest.php
@@ -285,7 +285,7 @@ public static function noop()
     {
     }
 
-    private function buildSchema(string $key = 'test', SchemaConfig $context = null): Schema
+    private function buildSchema(string $key = 'test', ?SchemaConfig $context = null): Schema
     {
         $schema = new Schema($key, $this->createSchemaContext());