Android : java.io.UnsupportedEncodingException: application/json; charset=UTF-8

I have a problem in this Android code snippet:

public Object post(String path, Object payload, Class<?> responseType) throws Exception {
    URI uri = buildUri(path);
    HttpPost request = new HttpPost(uri);
    addHeaders(request);
    request.setEntity(new StringEntity(
            serialize(payload),
            ContentType.APPLICATION_JSON));
    return op(request, responseType);
}

'StringEntity(java.lang.String, java.lang.String)' in 'org.apache.http.entity.StringEntity' cannot be applied to '(java.lang.String, org.apache.http.entity.ContentType)'

How can I correctly convert ContentType to String? I tried the use toString(), but got an error:

java.io.UnsupportedEncodingException: application/json; charset=UTF-8

I would be grateful for your help.

Thanks.