diff --git a/src/repository/api/ArcanistGitAPI.php b/src/repository/api/ArcanistGitAPI.php
index 6c6d2ac4..a0c0e909 100644
--- a/src/repository/api/ArcanistGitAPI.php
+++ b/src/repository/api/ArcanistGitAPI.php
@@ -598,6 +598,9 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
   }
 
   public function getCanonicalRevisionName($string) {
+    if (!$string) {
+      return null;
+    }
     $match = null;
 
     if (preg_match('/@([0-9]+)$/', $string, $match)) {