diff --git a/src/Flex.php b/src/Flex.php
index 6c53270a..f6481d34 100644
--- a/src/Flex.php
+++ b/src/Flex.php
@@ -162,6 +162,7 @@ class_exists(__NAMESPACE__.str_replace('/', '\\', substr($file, \strlen(__DIR__)
 
             $resolver = new PackageResolver($this->downloader);
 
+            $command = null;
             $commandObj = null;
             try {
                 $command = $input->getFirstArgument();
@@ -180,7 +181,7 @@ class_exists(__NAMESPACE__.str_replace('/', '\\', substr($file, \strlen(__DIR__)
                 $symfonyRequire = null;
             }
 
-            if (isset(self::$aliasResolveCommands[$command])) {
+            if (null !== $command && isset(self::$aliasResolveCommands[$command])) {
                 // When the command name is abbreviated (e.g. "req" for "require"), Composer
                 // activates plugins early to look up potential script commands, before the
                 // input has been bound to the command definition. In that case "packages"