diff --git a/hgext/patchbomb.py b/hgext/patchbomb.py
--- a/hgext/patchbomb.py
+++ b/hgext/patchbomb.py
@@ -139,6 +139,14 @@
     default=None,
 )
 
+class _filestrtobytesfacade(object):
+    def __init__(self, ui):
+        self._ui = ui
+    def write(self, data):
+        if not isinstance(data, bytes):
+            data = pycompat.bytesurl(data)
+        return self._ui.write(data)
+
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
 # be specifying the version(s) of Mercurial they are tested with, or
@@ -783,7 +791,8 @@
         if opts.get('test'):
             ui.status(_('displaying '), subj, ' ...\n')
             ui.pager('email')
-            generator = emailgen.Generator(ui, mangle_from_=False)
+            generator = emailgen.Generator(_filestrtobytesfacade(ui),
+                                           mangle_from_=False)
             try:
                 generator.flatten(m, 0)
                 ui.write('\n')
@@ -799,7 +808,8 @@
                 # Exim does not remove the Bcc field
                 del m['Bcc']
             fp = stringio()
-            generator = emailgen.Generator(fp, mangle_from_=False)
+            generator = emailgen.Generator(_filestrtobytesfacade(fp),
+                                           mangle_from_=False)
             generator.flatten(m, 0)
             sendmail(sender_addr, to + bcc + cc, fp.getvalue())