C: Use free () function to release double character pointer correctly

I have a function in which I need to implement the use of free () to free the memory that was used. My code is written this way:

void totalDePaisesVisitados(ALUMNO* array, int nu_Alumnos){
  int ArrayLenght = 0;
  int MaxPises = 70;
  int lastIndex = 0;
  char** stringAr = (char**) malloc(sizeof(char*)*MaxPises);
  for (int i = 0; i < MaxPises; i++) {
    stringAr[i] =  (char*) malloc(sizeof(char)*50);
  }
  for (int a = 0; a < nu_Alumnos; a++) {
    int numberPaises = array[a].pais->n_pais;
    //printf("%s %d\n","numero de paises", numberPaises);
    for (int b = 0; b < numberPaises; b++) {
      int enArreglo = 0;
      for (int c = 0; c < ArrayLenght+1; c++) {
        //printf("%s %s %s %s\n","Comparando: ",array[a].pais->nombre[b],"con: ",stringAr[c]);
        if (strcmp(array[a].pais->nombre[b],stringAr[c]) == 0) {
            enArreglo = 1;
            //printf("%s\n","Encontrado en el arreglo" );
        }
      }
      if (enArreglo == 0) {
          //printf("%s\n","ENCONTRADO PAIS DISTINTO" );
          stringAr[lastIndex] = array[a].pais->nombre[b];
          ArrayLenght++;
          lastIndex++;
      }
    }
  }
  printf("%s\n","#3");
  printf("%s %d\n","Numero de Paises visitados:",lastIndex);
  for (int i = 0; i < MaxPises; i++) {
    free(stringAr[i]);
  }
  free(stringAr);
}

The problem with this is that if I release the memory, an error appears:

  

malloc: *** error for object 0x7fe4356005d0: pointer being freed was   not allocated

What is the correct way to release the memory in this function?