diff --git a/specification/machinelearningservices/resource-manager/Microsoft.MachineLearningServices/preview/2021-03-01-preview/machineLearningServices.json b/specification/machinelearningservices/resource-manager/Microsoft.MachineLearningServices/preview/2021-03-01-preview/machineLearningServices.json
index d34f75ec32e7..43d70cc024d5 100644
--- a/specification/machinelearningservices/resource-manager/Microsoft.MachineLearningServices/preview/2021-03-01-preview/machineLearningServices.json
+++ b/specification/machinelearningservices/resource-manager/Microsoft.MachineLearningServices/preview/2021-03-01-preview/machineLearningServices.json
@@ -4379,13 +4379,13 @@
         },
         "notebookPreparationError": {
           "$ref": "#/definitions/NotebookPreparationError",
-          "description": "The error that occurs when preparing notebook."
+          "description": "The error that occurs when preparing notebook.",
+          "x-nullable": true
         }
       }
     },
     "NotebookPreparationError": {
       "type": "object",
-      "x-nullable": true,
       "properties": {
         "errorMessage": {
           "type": "string"