That is why. Your OS iage has to be the size of an established disk size. Try placing your kernel binary file inside a file that is the size of a 1.44MB disk image.You can do that with DD:
Code:
dd if=/dev/zero of=os.img bs=1024 count=1440
dd if=kernel.bin of=os.img conv=notrunc seek=0
The first command creates an image file (os.img) the size of a 1.44MB floppy (1474560 bytes).The second command takes the name of your binary kernel (kernel.bin is whatever name you used to save your binary kernel file as) and places it starting at the beginning of the image without truncating the image file. This should produce a disk with your kernel inside what appears to be a 1.44MB disk image.