mp3_wait_folder_sum() not working on ESP8266

[sp9wun]

Short code:

#include <SoftwareSerial.h>
#include <DFPlayer_Mini_Mp3.h>

SoftwareSerial mySerial(4, 5); // D1-RX, D2-TX

void setup () {
	Serial.begin (115200);
	mySerial.begin (9600);
	mp3_set_serial (mySerial);	//set softwareSerial for DFPlayer-mini mp3 module 
	mp3_set_volume (30);
  mp3_get_folder_sum(1);
  delay(1000);
//  int a = 0;
  int a = mp3_wait_folder_sum();
  Serial.print("Tracks: ");
  Serial.println(a);
}


//
void loop () {        
	mp3_play (1);
	delay (6000);
	mp3_next ();
	delay (6000);
	mp3_prev ();
	delay (6000);
	mp3_play (4);
	delay (6000);
}

Produces:

Exception (28):
epc1=0x4020245a epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000

ctx: cont 
sp: 3ffef480 end: 3ffef6c0 offset: 01a0

>>>stack>>>
3ffef620:  3ffee4a4 3ffee698 402028d0 3ffee6a0  
3ffef630:  3ffe84b0 00000005 00000005 402010a3  
3ffef640:  00000000 3ffee4f8 0000004e 40202478  
3ffef650:  40105228 00000000 0000004e 4020215f  
3ffef660:  3ffedea0 3ffee524 3ffee6a0 3ffee698  
3ffef670:  3fffdad0 3ffee470 3ffee5e4 402021b0  
3ffef680:  402010ae 000003e8 000003e8 40202244  
3ffef690:  3fffdad0 3ffee470 3ffee5e4 40201c31  
3ffef6a0:  feefeffe 00000000 3ffee690 40202914  
3ffef6b0:  feefeffe feefeffe 3ffee6a0 40100718  
<<<stack<<<

 ets Jan  8 2013,rst cause:2, boot mode:(3,6)

load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
v09f0c112
~ld
…

Tested on Wemos D1 R2 board.