diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -934,7 +934,7 @@
             name = os.path.join(tmproot, pre)
             if ext:
                 name += ext
-            f = open(name, "wb")
+            f = open(name, b"wb")
         else:
             fd, name = pycompat.mkstemp(prefix=pre + b'.', suffix=ext)
             f = os.fdopen(fd, "wb")