get_workflowSubscriptionId should return a SP.Guid

sharepoint/SharePoint.d.ts

@@ -8448,7 +8448,7 @@ declare module SP.WorkflowServices {
         /** Specifies the custom status set by workflow authors. */
         set_userStatus(value: string): string;
         /** Gets the unique identifier (GUID) of the subscription that instantiates the WorkflowInstance */
-        get_workflowSubscriptionId(): string;
+        get_workflowSubscriptionId(): SP.Guid;
         /** This method is internal and is not intended to be used in your code. */
         initPropertiesFromJson(parentNode: any): void;
 
@@ -11205,4 +11205,4 @@ declare module SP {
 
         get_Columns(): SP.JsGrid.ColumnInfo[];
     }
-}
+}