diff --git a/src/applications/differential/editor/DifferentialTransactionEditor.php b/src/applications/differential/editor/DifferentialTransactionEditor.php
--- a/src/applications/differential/editor/DifferentialTransactionEditor.php
+++ b/src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -249,6 +249,11 @@
           case DifferentialAction::ACTION_CLAIM:
             $object->setAuthorPHID($this->getActingAsPHID());
             return;
+          default:
+            throw new Exception(
+              pht(
+                'Differential action "%s" is not a valid action!',
+                $xaction->getNewValue()));
         }
         break;
     }