MockServer | Return simple list in HttpResponse

Question

We're calling an external API that return a simple List, not DTO/Json, like ResponseEntity<List<String>>

In my tests I'm trying to mock the response using mockServer but failed to return the result as List (just String as it not helped me):

mockServer.when(restTemplate.getForObject(any(), any()))
            .respond(HttpResponse.response("{\"40\", \"50\"}")
                    .withStatusCode(200)
                    .withDelay(TimeUnit.SECONDS, 1));

String user = "user";
String password = "password";
RestTemplate restTemplate = new RestTemplate();
restTemplate.getInterceptors().add(new BasicAuthenticationInterceptor(user, password));
List<String> retList = restTemplate.getForObject(URI, List.class);

I tried with [, ], {, }, none of them, double parenthesis but failed to return the response as List..

How can I do that correctly?

P.s. in the code all works well with the real API, response came as list as well. The issue is only in the tests, the error is:

"Could not extract response: no suitable HttpMessageConverter found for response type [interface java.util.List] and content type [application/octet-stream]"