diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -224,8 +224,12 @@
     minver = getattr(mod, 'minimumhgversion', None)
     if minver:
         curver = util.versiontuple(n=2)
+        extmin = util.versiontuple(minver, 2)
 
-        if None in curver or util.versiontuple(minver, 2) > curver:
+        if None in extmin:
+            extmin = (extmin[0] or 0, extmin[1] or 0)
+
+        if None in curver or extmin > curver:
             msg = _(
                 b'(third party extension %s requires version %s or newer '
                 b'of Mercurial (current: %s); disabling)\n'
diff --git a/tests/test-extension.t b/tests/test-extension.t
--- a/tests/test-extension.t
+++ b/tests/test-extension.t
@@ -1692,6 +1692,25 @@
   $ hg --config extensions.minversion=minversion3.py version 2>&1 | egrep '\(third'
   [1]
 
+Don't explode on py3 with a bad version number
+
+  $ cat > minversion4.py << EOF
+  > from mercurial import util
+  > util.version = lambda: b'3.5'
+  > minimumhgversion = b'3'
+  > EOF
+  $ hg --config extensions.minversion=minversion4.py version -v
+  Mercurial Distributed SCM (version 3.5)
+  (see https://mercurial-scm.org for more information)
+  
+  Copyright (C) 2005-* Olivia Mackall and others (glob)
+  This is free software; see the source for copying conditions. There is NO
+  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  
+  Enabled extensions:
+  
+    minversion  external  
+
 Restore HGRCPATH
 
   $ HGRCPATH=$ORGHGRCPATH