error 'MIFARE_Key' in 'class MFRC522' does not name a type

//RST           D9
//SDA(SS)      D10
//MOSI         D11
//MISO         D12
//SCK          D13

#include <SPI.h>
#include <MFRC522.h>

const int RST_PIN = 9;
const int SS_PIN = 10;

//Declaracion de cadena de caracteres
unsigned char data[16] = { 'T','E','S','T',' ','R','F','I','D',' ','M','F','R', '5','5','2'}; 
unsigned char *writeData = data; 
unsigned char *str;

MFRC522 mfrc522(SS_PIN, RST_PIN);

MFRC522::MIFARE_Key key;

void printArray(byte *buffer, byte bufferSize) {
   for (byte i = 0; i < bufferSize; i++) {
      Serial.print(buffer[i] < 0x10 ? " 0" : " ");
      Serial.print(buffer[i], HEX);
   }
}