diff --git a/src/applications/transactions/conduit/TransactionSearchConduitAPIMethod.php b/src/applications/transactions/conduit/TransactionSearchConduitAPIMethod.php
--- a/src/applications/transactions/conduit/TransactionSearchConduitAPIMethod.php
+++ b/src/applications/transactions/conduit/TransactionSearchConduitAPIMethod.php
@@ -388,6 +388,14 @@
       }
 
       $object = $all_types[$object_type]->newObject();
+      if (!$object) {
+        throw new Exception(
+          pht(
+            'In call to "transaction.search", specified "objectType" ("%s") '.
+            'exists but is not supported. Try with a supported value like %s.',
+            $object_type,
+            '"TASK"'));
+      }
     } else {
       $object = id(new PhabricatorObjectQuery())
         ->setViewer($viewer)