diff --git a/php/src/Google/Protobuf/Internal/GPBDecodeException.php b/php/src/Google/Protobuf/Internal/GPBDecodeException.php
index 6a143a2a0db04..7104109fab482 100644
--- a/php/src/Google/Protobuf/Internal/GPBDecodeException.php
+++ b/php/src/Google/Protobuf/Internal/GPBDecodeException.php
@@ -14,7 +14,7 @@ class GPBDecodeException extends \Exception
     public function __construct(
         $message,
         $code = 0,
-        \Exception $previous = null)
+        ?\Exception $previous = null)
     {
         parent::__construct(
             "Error occurred during parsing: " . $message,