diff --git a/src/Readline/Hoa/Protocol.php b/src/Readline/Hoa/Protocol.php
index 51d064e0..922ddcbf 100644
--- a/src/Readline/Hoa/Protocol.php
+++ b/src/Readline/Hoa/Protocol.php
@@ -87,7 +87,7 @@ public static function getInstance(): self
     protected function initialize()
     {
         $root = \dirname(__DIR__, 3);
-        $argv0 = \realpath($_SERVER['argv'][0]);
+        $argv0 = isset($_SERVER['argv'][0]) ? \realpath($_SERVER['argv'][0]) : false;
 
         $cwd =
             'cli' === \PHP_SAPI