diff --git a/src/AuthorizationHeaderBuilder.php b/src/AuthorizationHeaderBuilder.php
index ab105f0..c24d567 100644
--- a/src/AuthorizationHeaderBuilder.php
+++ b/src/AuthorizationHeaderBuilder.php
@@ -73,11 +73,11 @@ class AuthorizationHeaderBuilder
      *   The request for which to generate the authorization header.
      * @param \Acquia\Hmac\KeyInterface $key
      *   The key with which to sign the authorization header.
-     * @param \Acquia\Hmac\Digest\DigestInterface $digest
+     * @param \Acquia\Hmac\Digest\DigestInterface|null $digest
      *   The message digest to use when signing requests. Defaults to
      *   \Acquia\Hmac\Digest\Digest.
      */
-    public function __construct(RequestInterface $request, KeyInterface $key, DigestInterface $digest = null)
+    public function __construct(RequestInterface $request, KeyInterface $key, ?DigestInterface $digest = null)
     {
         $this->request = $request;
         $this->key     = $key;
diff --git a/src/Exception/MalformedRequestException.php b/src/Exception/MalformedRequestException.php
index 5aa80ff..33454cc 100644
--- a/src/Exception/MalformedRequestException.php
+++ b/src/Exception/MalformedRequestException.php
@@ -27,7 +27,7 @@ class MalformedRequestException extends InvalidRequestException
      * @param \Psr\Http\Message\RequestInterface|null $request
      *   The request.
      */
-    public function __construct($message = "", \Exception $previous = null, $code = 0, RequestInterface $request = null)
+    public function __construct($message = "", ?\Exception $previous = null, $code = 0, ?RequestInterface $request = null)
     {
         parent::__construct($message, $code, $previous);
         $this->request = $request;
diff --git a/src/Exception/MalformedResponseException.php b/src/Exception/MalformedResponseException.php
index 90bd4df..fea55c9 100644
--- a/src/Exception/MalformedResponseException.php
+++ b/src/Exception/MalformedResponseException.php
@@ -29,9 +29,9 @@ class MalformedResponseException extends InvalidRequestException
      */
     public function __construct(
         $message = "",
-        \Exception $previous = null,
+        ?\Exception $previous = null,
         $code = 0,
-        ResponseInterface $response = null
+        ?ResponseInterface $response = null
     ) {
         parent::__construct($message, $code, $previous);
         $this->response = $response;
diff --git a/src/RequestSigner.php b/src/RequestSigner.php
index 3452c07..09b1047 100644
--- a/src/RequestSigner.php
+++ b/src/RequestSigner.php
@@ -62,7 +62,7 @@ public function signRequest(RequestInterface $request, array $customHeaders = []
     /**
      * {@inheritDoc}
      */
-    public function getTimestampedRequest(RequestInterface $request, \DateTime $date = null)
+    public function getTimestampedRequest(RequestInterface $request, ?\DateTime $date = null)
     {
         if ($request->hasHeader('X-Authorization-Timestamp')) {
             return clone $request;
diff --git a/src/RequestSignerInterface.php b/src/RequestSignerInterface.php
index 1888b60..51cebe8 100644
--- a/src/RequestSignerInterface.php
+++ b/src/RequestSignerInterface.php
@@ -28,7 +28,7 @@ public function signRequest(RequestInterface $request, array $customHeaders = []
      *
      * @param \Psr\Http\Message\RequestInterface $request
      *   The request being signed.
-     * @param \DateTime
+     * @param \DateTime|null $date
      *   The date to timestamp the request with. Defaults to now.
      *
      * @return \Psr\Http\Message\RequestInterface $request
diff --git a/src/ResponseSigner.php b/src/ResponseSigner.php
index 017e392..ac5a8ea 100644
--- a/src/ResponseSigner.php
+++ b/src/ResponseSigner.php
@@ -36,11 +36,11 @@ class ResponseSigner implements ResponseSignerInterface
      *   The key with which to sign the response.
      * @param \Psr\Http\Message\RequestInterface $request
      *   The original response corresponding to the response being signed.
-     * @param \Acquia\Hmac\Digest\Digest $digest
+     * @param \Acquia\Hmac\Digest\DigestInterface|null $digest
      *   The digest with which to sign the response. Defaults to
      *   \Acquia\Hmac\Digest\Digest.
      */
-    public function __construct(KeyInterface $key, RequestInterface $request, DigestInterface $digest = null)
+    public function __construct(KeyInterface $key, RequestInterface $request, ?DigestInterface $digest = null)
     {
         $this->key = $key;
         $this->request = $request;
diff --git a/src/Symfony/HmacAuthenticationEntryPoint.php b/src/Symfony/HmacAuthenticationEntryPoint.php
index 65cdd0b..b963835 100644
--- a/src/Symfony/HmacAuthenticationEntryPoint.php
+++ b/src/Symfony/HmacAuthenticationEntryPoint.php
@@ -15,7 +15,7 @@ class HmacAuthenticationEntryPoint implements AuthenticationEntryPointInterface
     /**
      * {@inheritDoc}
      */
-    public function start(Request $request, AuthenticationException $authException = null)
+    public function start(Request $request, ?AuthenticationException $authException = null)
     {
         $response = new Response();
         $response->setStatusCode(401, $authException ? $authException->getMessage() : null);
diff --git a/src/Symfony/HmacToken.php b/src/Symfony/HmacToken.php
index 114b2ed..8388563 100644
--- a/src/Symfony/HmacToken.php
+++ b/src/Symfony/HmacToken.php
@@ -33,7 +33,7 @@ class HmacToken extends AbstractToken
      * @param \Symfony\Component\Security\Core\Role\RoleInterface[]|string[] $roles
      *   An array of roles.
      */
-    public function __construct(Request $request, KeyInterface $key = null, array $roles = [])
+    public function __construct(Request $request, ?KeyInterface $key = null, array $roles = [])
     {
         parent::__construct($roles);
 
diff --git a/test/Mocks/MockRequestAuthenticator.php b/test/Mocks/MockRequestAuthenticator.php
index b306716..622b4c0 100644
--- a/test/Mocks/MockRequestAuthenticator.php
+++ b/test/Mocks/MockRequestAuthenticator.php
@@ -33,7 +33,7 @@ class MockRequestAuthenticator extends RequestAuthenticator
      * @param int $timestamp
      *   An optional custom timestamp by which to compare requests.
      */
-    public function __construct(KeyLoaderInterface $keyLoader, AuthorizationHeaderInterface $authHeader = null, $timestamp = null)
+    public function __construct(KeyLoaderInterface $keyLoader, ?AuthorizationHeaderInterface $authHeader = null, $timestamp = null)
     {
         parent::__construct($keyLoader);