diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -936,6 +936,10 @@
         version = b''
     if isinstance(version, (list, tuple)):
         version = b'.'.join(pycompat.bytestr(o) for o in version)
+    else:
+        # version data should be bytes, but not all extensions are ported
+        # to py3.
+        version = stringutil.forcebytestr(version)
     return version
 
 
diff --git a/tests/test-extension.t b/tests/test-extension.t
--- a/tests/test-extension.t
+++ b/tests/test-extension.t
@@ -1399,12 +1399,20 @@
   > cmdtable = {}
   > command = registrar.command(cmdtable)
   > class Bogon(Exception): pass
+  > # NB: version should be bytes; simulating extension not ported to py3
+  > __version__ = '1.0.0'
   > @command(b'throw', [], b'hg throw', norepo=True)
   > def throw(ui, **opts):
   >     """throws an exception"""
   >     raise Bogon()
   > EOF
 
+Test extension without proper byteification of key attributes doesn't crash when
+accessed.
+
+  $ hg version -v --config extensions.throw=throw.py | grep '^ '
+    throw  external  1.0.0
+
 No declared supported version, extension complains:
   $ hg --config extensions.throw=throw.py throw 2>&1 | egrep '^\*\*'
   ** Unknown exception encountered with possibly-broken third-party extension throw