arduino + sim800l library + smsread problem

hello, I'm using Arduino UNO and sim800l module and "sim800l library". I can send SMS but I can't read sms. I'm using sim800l-library's example.

code:

/* this library is writing by  Cristian Steib.
 *      steibkhriz@gmail.com
 *  Designed to work with the GSM Sim800l,maybe work with SIM900L
 *  
 *     This library use SoftwareSerial, you can define de RX and TX pin
 *     in the header "Sim800l.h" ,by default the pin is RX=10 TX=11..
 *     be sure that gnd is attached to arduino too. 
 *     You can also change the other preferred RESET_PIN
 *    
 *     Esta libreria usa SoftwareSerial, se pueden cambiar los pines de RX y TX
 *     en el archivo header, "Sim800l.h", por defecto los pines vienen configurado en
 *     RX=10 TX=11.  
 *     Tambien se puede cambiar el RESET_PIN por otro que prefiera
 *     
 *    PINOUT: 
 *        _____________________________
 *       |  ARDUINO UNO >>>   SIM800L  |
 *        -----------------------------
 *            GND      >>>   GND
 *        RX  10       >>>   TX    
 *        TX  11       >>>   RX
 *       RESET 2       >>>   RST 
 *                 
 *   POWER SOURCE 4.2V >>> VCC
 *
 *    Created on: April 20, 2016
 *        Author: Cristian Steib
 *        
 *
*/

#include <Sim800l.h>
#include <SoftwareSerial.h> //is necesary for the library!! 
Sim800l Sim800l;  //to declare the library
String text;     // to almacenate the text of the sms
uint8_t index;   // to indicate the message to read.


void setup(){
	Serial.begin(9600); // only for debug the results . 
	Sim800l.begin(); // initializate the library. 
	index=1; // first position in the prefered memory storage. 
	text=Sim800l.readSms(index);   	
	Serial.println(text);

}

void loop(){
	//do nothing
}

Serial monitor shows this:
"please check attachment"

thank you