How to test POST request?

I have a problem with test my ktor application - I cannot do a post request correctly.

My code:

object AdminAuthHandlerTest : Spek({
    with(ktorTestEngine()) {
        describe("A sign-in endpoint") {
            lateinit var call: TestApplicationCall
            beforeGroup {
                call = handleRequest(HttpMethod.Post, "/api/admin/auth/sign-in") {
                    addHeader(HttpHeaders.Accept, ContentType.Text.Plain.contentType)
                    addHeader(HttpHeaders.ContentType, ContentType.Application.Json.contentType)
                    setBody(jsonAsString(AdminSignInRequest("admin", "admin123")))
                }
            }
            it("should return 200") {
                assertThat(call.response.status()).isEqualTo(HttpStatusCode.OK)
            }
        }
    }
})

I get 400 (Bad Request), but my request body is correct.

Do you have any idea what is wrong?