diff --git a/src/OpenTok/OpenTok.php b/src/OpenTok/OpenTok.php
index 8808d24..524dd26 100644
--- a/src/OpenTok/OpenTok.php
+++ b/src/OpenTok/OpenTok.php
@@ -1261,7 +1261,7 @@ public function dial($sessionId, $token, $sipUri, $options = [])
      *
      * @return void
      */
-    public function playDTMF(string $sessionId, string $digits, string $connectionId = null): void
+    public function playDTMF(string $sessionId, string $digits, ?string $connectionId = null): void
     {
         Validators::validateSessionIdBelongsToKey($sessionId, $this->apiKey);
         Validators::validateDTMFDigits($digits);
diff --git a/src/OpenTok/Util/Client.php b/src/OpenTok/Util/Client.php
index 0b4bb33..67a4dc1 100755
--- a/src/OpenTok/Util/Client.php
+++ b/src/OpenTok/Util/Client.php
@@ -753,7 +753,7 @@ public function dial($sessionId, $token, $sipUri, $options)
         return $sipJson;
     }
 
-    public function playDTMF(string $sessionId, string $digits, string $connectionId = null): void
+    public function playDTMF(string $sessionId, string $digits, ?string $connectionId = null): void
     {
         $route = sprintf('/v2/project/%s/session/%s/play-dtmf', $this->apiKey, $sessionId);
         if ($connectionId) {