diff -r 28626957395a -r d1e0b905c59d mercurial/hgweb/server.py
--- a/mercurial/hgweb/server.py	Tue Sep 25 22:46:18 2018 -0400
+++ b/mercurial/hgweb/server.py	Tue Sep 25 21:39:42 2018 -0400
@@ -174,8 +174,12 @@
         env[r'wsgi.errors'] = _error_logger(self)
         env[r'wsgi.multithread'] = isinstance(self.server,
                                              socketserver.ThreadingMixIn)
-        env[r'wsgi.multiprocess'] = isinstance(self.server,
-                                              socketserver.ForkingMixIn)
+        if util.safehasattr(socketserver, 'ForkingMixIn'):
+            env[r'wsgi.multiprocess'] = isinstance(self.server,
+                                                   socketserver.ForkingMixIn)
+        else:
+            env[r'wsgi.multiprocess'] = False
+
         env[r'wsgi.run_once'] = 0
 
         wsgiref.validate.check_environ(env)