DTH11 Sensor reading -999 on Arduino UNO and serial monitor

My Digital Temperature and Humidity sensor is reading -999 on both temperature and humidity, then reading the normal temperature, then reading -999 again on both temperature and humidity again. This pattern keeps going on. This is my code:

#include <dht.h>

dht DHT;

#define DHT11_PIN 7

void setup(){
  Serial.begin(9600);
}

void loop()
{
  int chk = DHT.read11(DHT11_PIN);
  Serial.print("Temperature = ");
  Serial.println(DHT.temperature);
  Serial.print("Humidity = ");
  Serial.println(DHT.humidity);
  delay(1000);
}

My Wiring:
DHT11 S - Arduino Uno pin 7
DHT11 VCC - Arduino 5V
DHT11 GND - Arduino GND

I hope someone can help.

I guess the pull-up resistor on the sensor's data signal is missing.