No allowed to create a startup USB when click on erase disk option

To reproduce bug:
Execute startup disk creator
Click erase disk option
Then, is showed that the free space in the disk is 0.0 and it is not allowed to burn the image.

Workaround:
When click the erase disk option, close startup disk creator application, then open again and the USB stick will be empty and ready for install ubuntu.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: usb-creator-common 0.2.38
ProcVersionSignature: Ubuntu 3.2.0-23.36-generic 3.2.14
Uname: Linux 3.2.0-23-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu4
Architecture: amd64
Date: Fri Apr 20 12:43:06 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Beta amd64 (20120328)
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: usb-creator
UpgradeStatus: No upgrade log present (probably fresh install)