arc land failing

[Simon Marlow]

I haven't been able to arc land in GHC for a while, it always complains 
about not being able to fast-forward, like this:

$ git pull --rebase
First, rewinding head to replay your work on top of it...
Applying: Mask to avoid uncaught ^C exceptions
$ arc land
Landing current branch 'ghci-mask'.
Switched to branch master. Updating branch...
Switched back to branch ghci-mask.
Exception
Command failed with error #128!
COMMAND
git pull --ff-only --no-stat

STDOUT
(empty)

STDERR
fatal: Not possible to fast-forward, aborting.

(Run with --trace for a full exception trace.)
[1]    11904 exit 1     arc land


Anyone able to look into this?

Cheers,
Simon