diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -17,6 +17,7 @@ from . import (
     bdiff,
     error,
     mpatch,
+    pycompat,
     util,
 )
 
@@ -61,6 +62,7 @@ class diffopts(object):
         }
 
     def __init__(self, **opts):
+        opts = pycompat.byteskwargs(opts)
         for k in self.defaults.keys():
             v = opts.get(k)
             if v is None:
diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -2213,7 +2213,7 @@ def difffeatureopts(ui, opts=None, untru
         buildopts['nobinary'] = get('nobinary', forceplain=False)
         buildopts['noprefix'] = get('noprefix', forceplain=False)
 
-    return mdiff.diffopts(**buildopts)
+    return mdiff.diffopts(**pycompat.strkwargs(buildopts))
 
 def diff(repo, node1=None, node2=None, match=None, changes=None,
          opts=None, losedatafn=None, prefix='', relroot='', copy=None):