validateResponse fails even though response is valid

I am using the vanilla validateAllResponses from express-openapi and it seems to always throw an error on me when using a response that returns a JSON. Why could that be happening?

Example YAML (OpenAPI 3.0) for that specific path:

/HealthCheck:
    get:
      operationId: healthCheck
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: object
                properties:
                  status:
                    type: string
                    enum:
                      - OK

validateAllResponses on response “{“status”: “OK”}” (without the outer quotation marks, of course) says: “Invalid response for status code 200: should be object”. What am I missing? Isn’t this a valid object?

[aperkaz]

In case someone stumbles on this later on, here is the working version for Json responses:

 function validateAllResponses(req: any, res: any, next: any) {
    const strictValidation = req.apiDoc['x-express-openapi-validation-strict']
      ? true
      : false;
    if (typeof res.validateResponse === 'function') {
      const send = res.json;
      res.json = function expressOpenAPISend(...args: any[]) {
        const onlyWarn = !strictValidation;
        if (res.get('x-express-openapi-validation-error-for') !== undefined) {
          return send.apply(res, args);
        }
        const body = args[0];
        let validation = res.validateResponse(res.statusCode, body);
        let validationMessage;
        if (validation === undefined) {
          validation = { message: undefined, errors: undefined };
        }
        if (validation.errors) {
          const errorList = Array.from(validation.errors)
            .map((_) => _.message)
            .join(',');
          validationMessage = `Invalid response for status code ${res.statusCode}: ${errorList}`;
          // Set to avoid a loop, and to provide the original status code
          res.set(
            'x-express-openapi-validation-error-for',
            res.statusCode.toString()
          );
        }
        if (onlyWarn || !validation.errors) {
          return send.apply(res, args);
        } else {
          res.status(500);
          return res.json({ error: validationMessage });
        }
      };
    }
    next();
  }

[malnor]

@JasonSome This was asked quite some time ago. But also had this problem and it is because it overwrites the res.send but not the res.json. So if you would change all the send to json it will work for all json responses but probably not any send responses. I changed mine to json, because I just respond with json.