diff --git a/src/TestTraits/DrushTestTrait.php b/src/TestTraits/DrushTestTrait.php
index 15134d9b47..514375a8fc 100644
--- a/src/TestTraits/DrushTestTrait.php
+++ b/src/TestTraits/DrushTestTrait.php
@@ -90,7 +90,7 @@ public function drush($command, array $args = [], array $options = [], $site_spe
      * @param string $value The option value (or empty)
      * @return string
      */
-    protected function convertKeyValueToFlag(string $key, string $value)
+    protected function convertKeyValueToFlag(string $key, ?string $value)
     {
         if (!isset($value)) {
             return "--$key";