setsockopt fails while kernel TLS option enabling

Trying to play with in kernel TLS on kernel 5.3 version (Fedora 30 and 31) but stuck even on enabling ULP:

//  tls.c 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#include <sys/types.h>     
#include <sys/socket.h>
#include <linux/tls.h>
#include <netinet/ip.h> 
#include <netinet/tcp.h>


int main() {
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock == -1) {
        perror("socket creation");
        exit (EXIT_FAILURE);
    }

    if (setsockopt(sock, SOL_TCP, TCP_ULP, "tls", sizeof("tls")) == -1 )   {
        perror("tls init");
        exit (EXIT_FAILURE);
    }



close (sock);
    return EXIT_SUCCESS;
}

[user@localhost ~]$ cat /proc/sys/net/ipv4/tcp_available_ulp
tls
[user@localhost ~]$ gcc tls.c -O0 -g
[user@localhost ~]$ lsmod | grep tls
[user@localhost ~]$ ./a.out 
tls init: No such file or directory
[user@localhost ~]$ lsmod | grep tls
[user@localhost ~]$ sudo ./a.out 
tls init: Unknown error 524
[user@localhost ~]$ lsmod | grep tls
tls                    57344  0
[user@localhost ~]$ ./a.out 
tls init: Unknown error 524
[user@localhost ~]$ 

I don't know where I am wrong. It looks so simple in kernel network docs:

User interface

Creating a TLS connection

First create a new TCP socket and set the TLS ULP.

sock = socket(AF_INET, SOCK_STREAM, 0);
setsockopt(sock, SOL_TCP, TCP_ULP, "tls", sizeof("tls"));

Setting the TLS ULP allows us to set/get TLS socket options.

I don't understand how I can enable in kernel TLS and what I'am doing wrong.