fatal: git-write-tree: not able to write tree

I'm trying to  use git-am to apply a patch series in a mailbox.

The first patch has a conflict, which I edit to fix, and and then invoke
git am --3way --interactive --signoff --utf8 --resolved

but it bails out with this:

drivers/acpi/thermal.c: unmerged (4829f067a3e7acfbeed8b230caac00b1ed4b8554)
drivers/acpi/thermal.c: unmerged (528d198c28512af1627cce481575f37a599c0fe8)
drivers/acpi/thermal.c: unmerged (db3bef1a3e51801128e7553f3e546c8272cc9ee1)
fatal: git-write-tree: not able to write tree

I've tried various incantations of git reset on the theory that there is some 
old state hanging around someplace, but have not been able to check in this 
file.

clues?

thanks,
-Len