MacPort Upgrade Failed

[frankwang@apollo-mac bin]$ sudo port upgrade outdated
--->  Computing dependencies for glib2
--->  Fetching archive for glib2
--->  Attempting to fetch glib2-2.32.0_0.darwin_11.x86_64.tbz2 from http://packages.macports.org/glib2
--->  Attempting to fetch glib2-2.32.0_0.darwin_11.x86_64.tbz2.rmd160 from http://packages.macports.org/glib2
Error: Target org.macports.archivefetch returned: Failed to verify signature for archive!
Log for glib2 is at: /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_ports_devel_glib2/glib2/main.log
Error: Unable to upgrade port: 1

To report a bug, see <http://guide.macports.org/#project.tickets>

Why and how to solve?