buildtarget creates bogus default

[Steve Kimball]

How can buildtarget say PAYLOAD_SIZE is 0x-20000?

===> NOTE: Option FALLBACK_SIZE using default value 0x20000
===> NOTE: Option ROM_SECTION_SIZE using default value 0x20000
===> NOTE: Option ROM_SIZE using default value 0x100000
===> NOTE: Option ROM_SECTION_OFFSET using default value 0x0
===> NOTE: Option PAYLOAD_SIZE using default value 0x-20000

Options.lb has
default ROM_IMAGE_SIZE = 131072 (0x20000)

Can someone help me with a high-level view of these sizes?

_     _      _
     |     |      | ROM Section Size?
     |     |      |
     |     |      --
     |     | Normal
     |     |        _
     |     |       | Payload
     |     |       |
     |     --      --
ROM  |     |       | Rom Section Size?
SIZE |     |       |
     |     |       ---
     |     |  Fallback
     |     |       --
     |     |       | Payload
     |     |       |
     --    --      --

Thanks for the help,
Steve