diff --git a/src/workflow/ArcanistFeatureWorkflow.php b/src/workflow/ArcanistFeatureWorkflow.php
--- a/src/workflow/ArcanistFeatureWorkflow.php
+++ b/src/workflow/ArcanistFeatureWorkflow.php
@@ -167,7 +167,7 @@
     }
 
     echo $stdout;
-    fprintf(STDERR, $stderr);
+    fprintf(STDERR, '%s', $stderr);
     return $err;
   }