diff --git a/src/SpecBaseObject.php b/src/SpecBaseObject.php
index 1de429b..673f876 100644
--- a/src/SpecBaseObject.php
+++ b/src/SpecBaseObject.php
@@ -391,7 +391,7 @@ public function __unset($name)
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      * @throws exceptions\UnresolvableReferenceException in case resolving a reference fails.
      */
-    public function resolveReferences(ReferenceContext $context = null)
+    public function resolveReferences(?ReferenceContext $context = null)
     {
         // avoid recursion to get stuck in a loop
         if ($this->_recursingReferences) {
diff --git a/src/SpecObjectInterface.php b/src/SpecObjectInterface.php
index a96aa74..31f9f8c 100644
--- a/src/SpecObjectInterface.php
+++ b/src/SpecObjectInterface.php
@@ -40,7 +40,7 @@ public function getErrors(): array;
     /**
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      */
-    public function resolveReferences(ReferenceContext $context = null);
+    public function resolveReferences(?ReferenceContext $context = null);
 
     /**
      * Set context for all Reference Objects in this object.
diff --git a/src/spec/Callback.php b/src/spec/Callback.php
index 06bbd58..dc2c414 100644
--- a/src/spec/Callback.php
+++ b/src/spec/Callback.php
@@ -133,7 +133,7 @@ public function getErrors(): array
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      * @throws UnresolvableReferenceException
      */
-    public function resolveReferences(ReferenceContext $context = null)
+    public function resolveReferences(?ReferenceContext $context = null)
     {
         if ($this->_pathItem !== null) {
             $this->_pathItem->resolveReferences($context);
diff --git a/src/spec/PathItem.php b/src/spec/PathItem.php
index 4b2debc..22e5b13 100644
--- a/src/spec/PathItem.php
+++ b/src/spec/PathItem.php
@@ -150,7 +150,7 @@ public function setReferenceContext(ReferenceContext $context)
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      * @throws \cebe\openapi\exceptions\UnresolvableReferenceException in case resolving a reference fails.
      */
-    public function resolveReferences(ReferenceContext $context = null)
+    public function resolveReferences(?ReferenceContext $context = null)
     {
         if ($this->_ref instanceof Reference) {
             $pathItem = $this->_ref->resolve($context);
diff --git a/src/spec/Paths.php b/src/spec/Paths.php
index e504a90..f55bafe 100644
--- a/src/spec/Paths.php
+++ b/src/spec/Paths.php
@@ -246,7 +246,7 @@ public function getIterator(): Traversable
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      * @throws UnresolvableReferenceException
      */
-    public function resolveReferences(ReferenceContext $context = null)
+    public function resolveReferences(?ReferenceContext $context = null)
     {
         foreach ($this->_paths as $key => $path) {
             if ($path === null) {
diff --git a/src/spec/Reference.php b/src/spec/Reference.php
index cda612a..81f07e8 100644
--- a/src/spec/Reference.php
+++ b/src/spec/Reference.php
@@ -64,7 +64,7 @@ class Reference implements SpecObjectInterface, DocumentContextInterface
      * @param string $to class name of the type referenced by this Reference
      * @throws TypeErrorException in case invalid data is supplied.
      */
-    public function __construct(array $data, string $to = null)
+    public function __construct(array $data, ?string $to = null)
     {
         if (!isset($data['$ref'])) {
             throw new TypeErrorException(
@@ -170,7 +170,7 @@ public function getContext() : ?ReferenceContext
      * If you call resolveReferences() make sure to replace the Reference with the resolved object first.
      * @throws UnresolvableReferenceException in case of errors.
      */
-    public function resolve(ReferenceContext $context = null)
+    public function resolve(?ReferenceContext $context = null)
     {
         if ($context === null) {
             $context = $this->getContext();
@@ -357,7 +357,7 @@ private function makeRelativePath($base, $path)
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      * @throws UnresolvableReferenceException
      */
-    public function resolveReferences(ReferenceContext $context = null)
+    public function resolveReferences(?ReferenceContext $context = null)
     {
         throw new UnresolvableReferenceException('Cyclic reference detected, resolveReferences() called on a Reference Object.');
     }
diff --git a/src/spec/Responses.php b/src/spec/Responses.php
index a6db447..9e1233d 100644
--- a/src/spec/Responses.php
+++ b/src/spec/Responses.php
@@ -236,7 +236,7 @@ public function getIterator(): Traversable
      * Resolves all Reference Objects in this object and replaces them with their resolution.
      * @throws UnresolvableReferenceException
      */
-    public function resolveReferences(ReferenceContext $context = null)
+    public function resolveReferences(?ReferenceContext $context = null)
     {
         foreach ($this->_responses as $key => $response) {
             if ($response instanceof Reference) {