diff --git a/src/SpecBaseObject.php b/src/SpecBaseObject.php
index ef93401..0e5b2f4 100644
--- a/src/SpecBaseObject.php
+++ b/src/SpecBaseObject.php
@@ -393,7 +393,7 @@ 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/PathItem.php b/src/spec/PathItem.php
index 6930e0a..083e702 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 f9b84d6..9196424 100644
--- a/src/spec/Reference.php
+++ b/src/spec/Reference.php
@@ -363,7 +363,7 @@ 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) {