diff --git a/app/controllers/submissions_api_controller.rb b/app/controllers/submissions_api_controller.rb
index ab56792782f..f4fed69ea4b 100644
--- a/app/controllers/submissions_api_controller.rb
+++ b/app/controllers/submissions_api_controller.rb
@@ -852,6 +852,9 @@ class SubmissionsApiController < ApplicationController
   # @argument grade_data[<student_id>][file_ids][] [Integer]
   #   See documentation for the comment[] arguments in the
   #   {api:SubmissionsApiController#update Submissions Update} documentation
+  # @argument grade_data[<student_id>][assignment_id] [Integer]
+  #   Specifies which assignment to grade.  This argument is not necessary when
+  #   using the assignment-specific endpoints.
   #
   # @example_request
   #