Initialize sim808 module

Error on trying to initialize the sim808
Here is the code I am using, it fails on the
//******** Initialize sim808 module *************

/*
### Get GPS data
1. This example is used to test SIM808 GPS/GPRS/GSM Shield's reading GPS data.
2. Open the SIM808_GetGPS example or copy these code to your project
3. Download and dial the function switch to Arduino
4. open serial helper
4. Place it outside, waiting for a few minutes and then it will send GPS data to serial
*/



#include <DFRobot_sim808.h>
#include <SoftwareSerial.h>

//#define PIN_TX 10
//#define PIN_RX 11
//SoftwareSerial mySerial(PIN_TX,PIN_RX);
//DFRobot_SIM808 sim808(&mySerial);//Connect RX,TX,PWR,

DFRobot_SIM808 sim808(&Serial);



void setup() {
//mySerial.begin(9600);
Serial.begin(9600);

//******** Initialize sim808 module *************
while(!sim808.init()) {
delay(1000);
Serial.print("Sim808 init error\r\n");
}

//************* Turn on the GPS power************
if( sim808.attachGPS())
Serial.println("Open the GPS power success");
else
Serial.println("Open the GPS power failure");

}

void loop() {
//************** Send the coordinates(latitude&lonitude)in the Serial Monitor *******************
if (sim808.getGPS()) {

Serial.print("latitude :");
Serial.println(sim808.GPSdata.lat);
Serial.print("longitude :");
Serial.println(sim808.GPSdata.lon);

}

}