diff --git a/src/workflow/ArcanistDiffWorkflow.php b/src/workflow/ArcanistDiffWorkflow.php
--- a/src/workflow/ArcanistDiffWorkflow.php
+++ b/src/workflow/ArcanistDiffWorkflow.php
@@ -1947,7 +1947,7 @@
           $result[$key] = $new;
           continue;
         } else {
-          if (!strlen(trim($value))) {
+          if ($value === null || !strlen(trim($value))) {
             // Ignore empty fields.
             continue;
           }