"Cant read memory" error

[Cbomb]

This code works but retuns an error saying:
The instruction at "0x6f746172" referenced memory at "0x6f746172". The memory could not be "read".
(this error seems to occur at the very end of the code).

#include <windows.h>
#include <stdio.h>

int main()
{
	char s_username;
	unsigned long i_length;
	GetUserName (&s_username, &i_length);
	printf(&s_username);
	return 0;
}

Why is that?