diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -1130,10 +1130,10 @@ class ui(object):
 
         try:
             pager = subprocess.Popen(
-                command, shell=shell, bufsize=-1,
+                procutil.tonativestr(command), shell=shell, bufsize=-1,
                 close_fds=procutil.closefds, stdin=subprocess.PIPE,
                 stdout=procutil.stdout, stderr=procutil.stderr,
-                env=procutil.shellenviron(env))
+                env=procutil.tonativeenv(procutil.shellenviron(env)))
         except OSError as e:
             if e.errno == errno.ENOENT and not shell:
                 self.warn(_("missing pager command '%s', skipping pager\n")
diff --git a/mercurial/utils/procutil.py b/mercurial/utils/procutil.py
--- a/mercurial/utils/procutil.py
+++ b/mercurial/utils/procutil.py
@@ -320,10 +320,19 @@ def shellenviron(environ=None):
 if pycompat.iswindows:
     def shelltonative(cmd, env):
         return platform.shelltocmdexe(cmd, shellenviron(env))
+
+    tonativestr = encoding.strfromlocal
 else:
     def shelltonative(cmd, env):
         return cmd
 
+    tonativestr = pycompat.identity
+
+def tonativeenv(env):
+    '''convert the environment from bytes to strings suitable for Popen(), etc.
+    '''
+    return pycompat.rapply(tonativestr, env)
+
 def system(cmd, environ=None, cwd=None, out=None):
     '''enhanced shell command execution.
     run with environment maybe modified, maybe in different dir.