does not match "operator=" (operand types are "String" and "void")

I'm querying a web page that returns a JSON string using a NodeMCU ESP8266 microcontroller. The response of the web page looks like this:

{"1":true,"2":false,"3":false,"4":true,"5":true,"6":false,"7":false,"8":false}

The code I am using looks like this:

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
#include <ArduinoJson.h>
String payload = "";
const char* ssid = "ssid";
const char* password = "password";
String url = "example.com/data.json";
void setup() {
  Serial.begin(115200);
  delay(2000); while (!Serial);

  WiFi.begin(ssid, password);
  while (WiFi.status() !=WL_CONNECTED){
    delay(500);
    Serial.print(".");
  }
  Serial.println(WiFi.localIP());
  pinMode(D0, OUTPUT);
}
void loop() {
  StaticJsonBuffer<100> jsonBuffer;
  delay(5000);
  HTTPClient http;
  http.begin(url);
  int httpCode = http.GET();
  Serial.println(httpCode);
  Serial.println(http.getString());
  if (httpCode > 0) {
    payload = http.getString();
  }
  http.end();
  JsonObject& root = jsonBuffer.parseObject(payload);
  Serial.println(payload);
  if(!root.success()) {
    Serial.println("parseObject() failed");
  }
  if(root["1"] == true) {
    Serial.println("true");
  }
  digitalWrite(D0, !digitalRead(D0));
}

I believe the reason it fails to parse is that the payload variable ends with a trailing newline. payload = payload.trim();then i tried but then i got no match for 'operator=' (operand types are 'String' and 'void')so i tried the payload = payload.replace("\n,"");same problem and then i tried payload = String(payload);again and failed. What am I doing wrong?