When reading values from DHT21 sensor I get NaN, but sometimes real value too

Hello everyone, I hooked up the DHT21 sensor to my arduino UNO and I am trying to display data from the sensor to the monitor of my pc. But I get NaN and sometimes a value(when reading temperature). I checked the connections, but they seem fine. Also I am powering with 5v. I will apply picture of the results.
This is my code by now:

#include <Servo.h>
#include <DHT.h>

#define DHTPIN 7
#define DHTTYPE DHT21
DHT dht(DHTPIN, DHTTYPE);
float temperature;

Servo windowServo;
int position = 0;


void setup(){
  Serial.begin(9600);   
  Serial.println("hello");
  windowServo.attach(14);
  }

  void loop(){
    temperature = dht.readTemperature();
    Serial.print("Temperature now: ");
    Serial.println(temperature);
    if(position == 0){
      position = 45;
      windowServo.write(position);
    }
    else{
      position = 0;
      windowServo.write(position);
      }
    delay(2000);
    }

The pullup (5k range +/- ) is installed ?

If you only get occasional nan readings then you can filter them out easily. But as @knut_ny says a pull-up resostor is required!