diff -r df44bac9413d -r 33350debb480 mercurial/patch.py
--- a/mercurial/patch.py	Tue Feb 16 15:37:19 2021 +0530
+++ b/mercurial/patch.py	Tue Feb 16 15:44:51 2021 +0530
@@ -20,6 +20,7 @@
 from .i18n import _
 from .node import (
     hex,
+    nullhex,
     short,
 )
 from .pycompat import open
@@ -3099,11 +3100,13 @@
 
     ctx1, fctx1, path1, flag1, content1, date1 = data1
     ctx2, fctx2, path2, flag2, content2, date2 = data2
+    index1 = _gitindex(content1) if path1 in ctx1 else nullhex
+    index2 = _gitindex(content2) if path2 in ctx2 else nullhex
     if binary and opts.git and not opts.nobinary:
         text = mdiff.b85diff(content1, content2)
         if text:
             header.append(
-                b'index %s..%s' % (_gitindex(content1), _gitindex(content2))
+                b'index %s..%s' % (index1, index2)
             )
         hunks = ((None, [text]),)
     else:
@@ -3114,8 +3117,8 @@
             header.append(
                 b'index %s..%s %s'
                 % (
-                    _gitindex(content1)[0 : opts.index],
-                    _gitindex(content2)[0 : opts.index],
+                    index1[0 : opts.index],
+                    index2[0 : opts.index],
                     _gitmode[flag],
                 )
             )