How can I print a single letter from the stack using int 0x80?

I have written a simple Assembly program (Intel syntax / NASM & ld) to print a single character using the interruption 80h (0x80) on Linux. It works fine if I specify an address in memory where that character is located at, but it doesn't seem to work using the stack to store it. I'm setting ECX (the register that stores the address of the string to be printed) to the value of ESP after pushing 0x41 (hex code for the capital letter A) to the stack. It compiles fine but does nothing when I run it.

Below we have the code that worked and the code that didn't.

Works:

global _start   ;entry point
section .data
a: db "A"
section .text
_start:
    mov eax, 4  ;sys_write
    mov ebx, 1  ;stdout
    mov ecx, a  ;address of that char
    mov edx, 1  ;length of the string
    int 0x80

Doesn't work:

global _start   ;entry point
section .text
_start:
    mov eax, 4  ;sys_write
    mov ebx, 1  ;stdout
    push 0x00000041
    mov ecx, esp
    mov edx, 1  ;length of the string
    int 0x80

Any thoughts? I'm using the following script to compile, link and run the application:

nasm -felf64 main.s -o main.o
ld main.o -o main
./main

PS: I do know I'm using ELF64 as the output format for NASM. If I try ELF32, ld doesn't work. That's irrelevant in this situation.