RAMdisk installation -> boot overlaps partition

I would like to install NetBSD on my disk with the ramdisk kernel (sysinst). 
But after configuration of my disk, I got this error:

disklabel: boot overlaps used partition a
disklabel: warning, boot overlaps partition c, marked as FS_BOOT
disklabel: cannot install boot program

I tried to increase the start cylinder to give the installboot some more space. But even after 10MB,
the message doesn't go away. I'm little bit confused. Can somebody give me a hint?

Best Regards
Stephan