diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1364,14 +1364,14 @@
 
         extraextensions = []
         for opt in self._extraconfigopts:
-            section, key = _sys2bytes(opt).split(b'.', 1)
+            section, key = opt.split('.', 1)
             if section != 'extensions':
                 continue
-            name = key.split(b'=', 1)[0]
+            name = key.split('=', 1)[0]
             extraextensions.append(name)
 
         if extraextensions:
-            env['HGTESTEXTRAEXTENSIONS'] = b' '.join(extraextensions)
+            env['HGTESTEXTRAEXTENSIONS'] = ' '.join(extraextensions)
 
         # LOCALIP could be ::1 or 127.0.0.1. Useful for tests that require raw
         # IP addresses.
diff --git a/tests/test-run-tests.t b/tests/test-run-tests.t
--- a/tests/test-run-tests.t
+++ b/tests/test-run-tests.t
@@ -1956,9 +1956,12 @@
   $ cat << EOF >> test-config-opt.t
   >   $ hg init test-config-opt
   >   $ hg -R test-config-opt purge
+  >   $ echo "HGTESTEXTRAEXTENSIONS: \$HGTESTEXTRAEXTENSIONS"
+  >   HGTESTEXTRAEXTENSIONS: purge
   > EOF
 
-  $ rt --extra-config-opt extensions.purge= test-config-opt.t
+  $ rt --extra-config-opt extensions.purge= \
+  >    --extra-config-opt not.an.extension=True test-config-opt.t
   running 1 tests using 1 parallel processes 
   .
   # Ran 1 tests, 0 skipped, 0 failed.