diff --git a/lib/Client.php b/lib/Client.php
index b6bcd4a..375d059 100644
--- a/lib/Client.php
+++ b/lib/Client.php
@@ -217,22 +217,22 @@ class Client
     /**
      * Initialize the client.
      *
-     * @param string $host           the base url (e.g. https://api.sendgrid.com)
-     * @param array  $headers        global request headers
-     * @param string $version        api version (configurable) - this is specific to the SendGrid API
-     * @param array  $path           holds the segments of the url path
-     * @param array  $curlOptions    extra options to set during curl initialization
-     * @param bool   $retryOnLimit   set default retry on limit flag
-     * @param bool   $verifySSLCerts set default verify certificates flag
+     * @param string $host           The base url (e.g. https://api.sendgrid.com)
+     * @param array  $headers        Global request headers
+     * @param string $version        Api version (configurable) - this is specific to the SendGrid API
+     * @param array  $path           Holds the segments of the url path
+     * @param array  $curlOptions    Extra options to set during curl initialization
+     * @param bool   $retryOnLimit   Set default retry on limit flag
+     * @param bool   $verifySSLCerts Set default verify certificates flag
      */
     public function __construct(
-        $host,
-        $headers = null,
-        $version = null,
-        $path = null,
-        $curlOptions = null,
-        $retryOnLimit = false,
-        $verifySSLCerts = true
+        string $host,
+        ?array $headers = null,
+        ?string $version = null,
+        ?array $path = null,
+        ?array $curlOptions = null,
+        bool $retryOnLimit = false,
+        bool $verifySSLCerts = true
     ) {
         $this->host = $host;
         $this->headers = $headers ?: [];
@@ -263,7 +263,7 @@ public function getHost()
     public function setHost(string $host)
     {
         $this->host = $host;
-        
+
         return $this;
     }
 
@@ -364,7 +364,7 @@ public function setIsConcurrentRequest($isConcurrent)
      *
      * @return string
      */
-    private function buildUrl($queryParams = null)
+    private function buildUrl(?array $queryParams = null)
     {
         $path = '/' . implode('/', $this->path);
         if (isset($queryParams)) {
@@ -385,7 +385,7 @@ private function buildUrl($queryParams = null)
      *
      * @return array
      */
-    private function createCurlOptions($method, $body = null, $headers = null)
+    private function createCurlOptions($method, ?array $body = null, ?array $headers = null)
     {
         $options = [
                 CURLOPT_RETURNTRANSFER => true,
@@ -508,7 +508,7 @@ private function retryRequest(array $responseHeaders, $method, $url, $body, $hea
      *
      * @throws InvalidRequest
      */
-    public function makeRequest($method, $url, $body = null, $headers = null, $retryOnLimit = false)
+    public function makeRequest($method, $url, ?array $body = null, ?array $headers = null, $retryOnLimit = false)
     {
         $channel = curl_init($url);
 
@@ -604,7 +604,7 @@ public function makeAllRequests(array $requests = [])
      *
      * @return Client object
      */
-    public function _($name = null)
+    public function _(?string $name = null)
     {
         if (isset($name)) {
             $this->path[] = $name;
diff --git a/test/unit/MockClient.php b/test/unit/MockClient.php
index b1c897a..4b45433 100644
--- a/test/unit/MockClient.php
+++ b/test/unit/MockClient.php
@@ -10,7 +10,7 @@ class MockClient extends Client
     public $requestHeaders;
     public $url;
 
-    public function makeRequest($method, $url, $requestBody = null, $requestHeaders = null, $retryOnLimit = false)
+    public function makeRequest($method, $url, ?array $requestBody = null, ?array $requestHeaders = null, $retryOnLimit = false)
     {
         $this->requestBody = $requestBody;
         $this->requestHeaders = $requestHeaders;