Missing #/definitions/HttpError in Swashbuckle implementation

Hi,
I am working on a code base which is generating a swagger document correctly.

All of the endpoints are having HTTP responses which is referencing #/definitions/HttpError. However HTTPError is missing from my swagger JSON documentation. 

It would be of great help if someone could please guide me to fix the same. I want to include HTTPError in the "definitions" element of the json doc.

  "/User": {
      "get": {
        "tags": [
          "user"
        ],
        "summary": "Get the user information",
        "description": "User",
        "operationId": "User_Get_GET",
        "consumes": [],
        "produces": [
          "application/json",
          "text/json",
          "text/html",
          "application/octet-stream",
          "application/xml",
          "text/xml"
        ],
        "responses": {
          "200": {
            "description": "Successfully returns the system information",
            "schema": {
              "$ref": "#/definitions/SystemInfoDto"
            }
          },
          "403": {
            "description": "Unauthorized",
            "schema": {
              "$ref": "#/definitions/HttpError"
            }
          },
          "404": {
            "description": "Not Found",
            "schema": {
              "$ref": "#/definitions/HttpError"
            }
          },
          "409": {
            "description": "Another user has modified this record, please re-retrieve and try again",
            "schema": {
              "$ref": "#/definitions/HttpError"
            }
          }
        }
      }
    },