strlen in assembly

I made my own implementation of strlen in assembly, but it doesn't return the correct value. It returns the string length + 4. Consequently. I don't see why.. and I hope any of you do...

Assembly source:

section .text
    [GLOBAL stringlen:] ; C function

stringlen:  
    push ebp
    mov ebp, esp        ; setup the stack frame

    mov ecx, [ebp+8]

    xor eax, eax        ; loop counter


startLoop:
    xor edx, edx
    mov edx, [ecx+eax]
    inc eax

    cmp edx, 0x0 ; null byte    
    jne startLoop
end:    
    pop ebp

    ret

And the main routine:

#include <stdio.h>

extern int stringlen(char *);

int main(void)
{
  printf("%d", stringlen("h"));

  return 0;
}

Thanks