[Misiur]

Show us this whole function. Probably it's something like

pawn Код:

new File:file = fopen("blah.txt", io_read),
 // (...)
return (fwrite(file, str), fclose(file));

while you should check the handle:

pawn Код:

new File:file = fopen("blah.txt", io_read),
if(!file) {
    return false; //or something
}
 // (...)
return (fwrite(file, str), fclose(file));