diff --git a/src/workflow/ArcanistWorkflow.php b/src/workflow/ArcanistWorkflow.php
--- a/src/workflow/ArcanistWorkflow.php
+++ b/src/workflow/ArcanistWorkflow.php
@@ -1196,6 +1196,12 @@
     $future = $conduit->callMethod('differential.querydiffs', $params);
     $diff = head($future->resolve());
 
+    if ($diff == null) {
+      throw new Exception(
+        pht("Couldn't find a revision or diff that matches the given ID")
+      );
+    }
+
     $changes = array();
     foreach ($diff['changes'] as $changedict) {
       $changes[] = ArcanistDiffChange::newFromDictionary($changedict);