"Debug Assertion Failed" with fseek!!!

[dubae524]

#include <fstream.h>
#include <stdlib.h>
#include <iostream.h>
#include <stdio.h>

void main(void) {
FILE *testafile;
char myfilechar;
testafile = fopen("D:\testfile.fil", "r");
myfilechar = (char) fseek(testafile, 4, 0);
fclose(testafile);
printf("%c", myfilechar);
}

When I was using fseek, an assertion error came up. Can someone help me? (I was using MSVC++6 Std, problems were fseek.c, line 146, and str != NULL.)

This is a strange little problem, so I would like some help.