adt-run failed with "UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 46" when stderr contains non-ascii charaters

When stderr contains non-ascii characters adt-run fails with:

adt-run: trace: & dsc0t-build: - - - - - - - - - - results - - - - - - - - - -
adt-run: unexpected, exceptional, error:
Traceback (most recent call last):
  File "/usr/bin/adt-run", line 1860, in main
    process_actions()
  File "/usr/bin/adt-run", line 1836, in process_actions
    run_tests(stanzas, act.tests_tree)
  File "/usr/bin/adt-run", line 1095, in run_tests
    t.run(tree)
  File "/usr/bin/adt-run", line 1194, in run
    t.reportfail('status: %d, stderr: %s' % (rc, l))
  File "/usr/bin/adt-run", line 1119, in reportfail
    report(t.what, 'FAIL ' + m)
  File "/usr/bin/adt-run", line 167, in report
    preport('%-20s %s' % (tname, result))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 46: ordinal not in range(128)

In this particular test, the content of stderr was:
"""
gitest.c: In function ‘main’:
gitest.c:10:5: warning: ‘g_type_init’ is deprecated (declared at /usr/include/glib-2.0/gobject/gtype.h:669) [-Wdeprecated-declarations]
"""
(note the unicode quotes)

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: autopkgtest 2.2.3ubuntu3
ProcVersionSignature: Ubuntu 3.7.0-7.15-generic 3.7.0
Uname: Linux 3.7.0-7-generic x86_64
ApportVersion: 2.8-0ubuntu1
Architecture: amd64
Date: Wed Jan 16 14:44:48 2013
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: autopkgtest
UpgradeStatus: Upgraded to raring on 2012-01-31 (350 days ago)