Writing to allocated memory with sbrk results in segfault

Question

I'm trying to allocate memory with sys_brk and here is the program:

BYTES_TO_ALLOCATE equ 0x08

section .text
    global _start

_start:
    mov rax, 12 ;sys_brk number
    mov rdi, BYTES_TO_ALLOCATE
    syscall

    mov cl, 0x00 ;setting the value I need
    mov [rax], byte 0x01 ;SegFault
    mov rax, 60
    syscall

As specified in the linux manual

sbrk() increments the program's data space by increment bytes. Calling sbrk() with an increment of 0 can be used to find the current location of the program break.

I segfaulted when runnning the program. The register content before the segfaulted mov was:

rax            0x401000 4198400
rbx            0x0      0
rcx            0x40008c 4194444 

I also tried to decrement the rax value as follows:

BYTES_TO_ALLOCATE equ 0x08

_start:
    mov rax, 12 ;sys_brk number
    mov rdi, BYTES_TO_ALLOCATE
    syscall

    mov rbx, rax
    dec rbx
    mov [rbx], byte 0x01 ;Again SegFault
    mov rax, 60
    syscall

Now, I'm kind of confused how to use sys_brk return value. I took it from here. I also tried to use value in rcx but segfaulted anyway.