diff --git a/src/applications/differential/engineextension/DifferentialMailEngineExtension.php b/src/applications/differential/engineextension/DifferentialMailEngineExtension.php
index 24aa9fb329..29019b9e73 100644
--- a/src/applications/differential/engineextension/DifferentialMailEngineExtension.php
+++ b/src/applications/differential/engineextension/DifferentialMailEngineExtension.php
@@ -53,7 +53,7 @@ final class DifferentialMailEngineExtension
       } else {
         $reviewers[] = $reviewer_phid;
         if ($reviewer->isBlocking()) {
-          $reviewers[] = $blocking;
+          $blocking[] = $reviewer_phid;
         }
       }
     }
diff --git a/src/applications/metamta/stamp/PhabricatorPHIDMailStamp.php b/src/applications/metamta/stamp/PhabricatorPHIDMailStamp.php
index 575ad16f6a..ea7233fe5b 100644
--- a/src/applications/metamta/stamp/PhabricatorPHIDMailStamp.php
+++ b/src/applications/metamta/stamp/PhabricatorPHIDMailStamp.php
@@ -15,6 +15,15 @@ final class PhabricatorPHIDMailStamp
       return null;
     }
 
+    // TODO: This recovers from a bug where blocking reviewers were serialized
+    // incorrectly into the flat mail stamp list in the worker queue as arrays.
+    // It can be removed some time after February 2018.
+    foreach ($value as $key => $v) {
+      if (is_array($v)) {
+        unset($value[$key]);
+      }
+    }
+
     $viewer = $this->getViewer();
     $handles = $viewer->loadHandles($value);