Akka路线TestKit不能unmarshall响应作为串

我有以下试验：

 "fail if date is wrongly formatted" in {
    val endpoint = s"/api/prof?date=wrongdate"
    Get(endpoint) ~> check {
      status shouldBe StatusCodes.BadRequest
      val resp = responseAs[String]
      resp shouldBe "could not be parsed"
    }
 }

然而，测试失败如下：

Could not unmarshal response to type 'java.lang.String' for `responseAs` assertion: akka.http.scaladsl.unmarshalling.Unmarshaller$UnsupportedContentTypeException: Unsupported Content-Type [Some(text/plain; charset=UTF-8)], supported: application/json

Response was: HttpResponse(400 Bad Request,List(),HttpEntity.Strict(text/plain; charset=UTF-8,106 bytes total),HttpProtocol(HTTP/1.1))

我怎么能得到响应的身体作为一个字符串？