diff --git a/src/Composer/Util/Git.php b/src/Composer/Util/Git.php
index 67d42b7c2600..16d424aac418 100644
--- a/src/Composer/Util/Git.php
+++ b/src/Composer/Util/Git.php
@@ -336,6 +336,10 @@ private function isAuthenticationFailure($url, &$match)
         );
 
         $errorOutput = $this->process->getErrorOutput();
+        if (null === $errorOutput) {
+            return false;
+        }
+
         foreach ($authFailures as $authFailure) {
             if (strpos($errorOutput, $authFailure) !== false) {
                 return true;
diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index 7d7e6546eb75..8b6de878a3b4 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -361,7 +361,7 @@ public function splitLines($output)
     /**
      * Get any error output from the last command
      *
-     * @return string
+     * @return ?string
      */
     public function getErrorOutput()
     {