diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -224,7 +224,7 @@
     minver = getattr(mod, 'minimumhgversion', None)
     if minver:
         curver = util.versiontuple(n=2)
-        extmin = util.versiontuple(minver, 2)
+        extmin = util.versiontuple(stringutil.forcebytestr(minver), 2)
 
         if None in extmin:
             extmin = (extmin[0] or 0, extmin[1] or 0)
diff --git a/tests/test-extension.t b/tests/test-extension.t
--- a/tests/test-extension.t
+++ b/tests/test-extension.t
@@ -1692,12 +1692,13 @@
   $ hg --config extensions.minversion=minversion3.py version 2>&1 | egrep '\(third'
   [1]
 
-Don't explode on py3 with a bad version number
+Don't explode on py3 with a bad version number (both str vs bytes, and not enough
+parts)
 
   $ cat > minversion4.py << EOF
   > from mercurial import util
   > util.version = lambda: b'3.5'
-  > minimumhgversion = b'3'
+  > minimumhgversion = '3'
   > EOF
   $ hg --config extensions.minversion=minversion4.py version -v
   Mercurial Distributed SCM (version 3.5)