pbuilder fails when trying to mount /proc

Attempting to setup pbuilder for oneiric on oneiric:

pbuilder-dist oneiric create
<snip>
: Configuring build-essential...
I: Configuring libc-bin...
I: Base system installed successfully.
I: debootstrap finished
I: copying local configuration
I: Installing apt-lines
I: Refreshing the base.tgz
I: upgrading packages
I: mounting /proc filesystem
mount: /proc already mounted or /var/cache/pbuilder/build//29075/proc busy
mount: according to mtab, /proc is mounted on /proc
W: Aborting with an error
I: cleaning the build env
I: removing directory /var/cache/pbuilder/build//29075 and its subdirectories
rmdir: failed to remove `/var/cache/pbuilder/build//29075/proc': Device or resource busy
rmdir: failed to remove `/var/cache/pbuilder/build//29075': Directory not empty

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: pbuilder 0.199+nmu3ubuntu1
ProcVersionSignature: Ubuntu 3.0.0-5.6-generic 3.0.0-rc7
Uname: Linux 3.0.0-5-generic x86_64
Architecture: amd64
Date: Sun Jul 17 02:30:20 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
PackageArchitecture: all
SourcePackage: pbuilder
UpgradeStatus: Upgraded to oneiric on 2011-06-17 (29 days ago)