diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -890,7 +890,7 @@ def debugbuilddag(ui, repo, text,
         # we don't want to fail in merges during buildup
         os.environ['HGMERGE'] = 'internal:local'
 
-    def writefile(fname, text, fmode="w"):
+    def writefile(fname, text, fmode="wb"):
         f = open(fname, fmode)
         try:
             f.write(text)
@@ -925,7 +925,7 @@ def debugbuilddag(ui, repo, text,
                 merge(ui, repo, node=p2)
 
             if mergeable_file:
-                f = open("mf", "r+")
+                f = open("mf", "rb+")
                 try:
                     lines = f.read().split("\n")
                     lines[id * linesperrev] += " r%i" % id
@@ -935,7 +935,7 @@ def debugbuilddag(ui, repo, text,
                     f.close()
 
             if appended_file:
-                writefile("af", "r%i\n" % id, "a")
+                writefile("af", "r%i\n" % id, "ab")
 
             if overwritten_file:
                 writefile("of", "r%i\n" % id)
diff --git a/tests/test-debugbuilddag b/tests/test-debugbuilddag
--- a/tests/test-debugbuilddag
+++ b/tests/test-debugbuilddag
@@ -13,6 +13,8 @@ cd repo
 hg debugbuilddag '+2:f +3:p2 @temp <f+4 @default /p2 +2' -q -oa
 echo -- dag
 hg debugdag -t -b
+echo -- tip
+hg id
 echo -- glog
 hg glog --template '{rev}: {desc} [{branches}] @ {date}\n'
 echo -- glog of
@@ -35,6 +37,8 @@ cd repo
 hg debugbuilddag '+2:f +3:p2 @temp <f+4 @default /p2 +2' -q -mn
 echo -- dag
 hg debugdag -t -b
+echo -- tip
+hg id
 echo -- glog
 hg glog --template '{rev}: {desc} [{branches}] @ {date}\n'
 echo -- glog mf
diff --git a/tests/test-debugbuilddag.out b/tests/test-debugbuilddag.out
--- a/tests/test-debugbuilddag.out
+++ b/tests/test-debugbuilddag.out
@@ -4,6 +4,8 @@
 +3:p2
 @temp*f+3
 @default*/p2+2:tip
+-- tip
+f96e381c614c tip
 -- glog
 @  11: r11 [] @ 11.00
 |
@@ -101,6 +103,8 @@ r11
 +3:p2
 @temp*f+3
 @default*/p2+2:tip
+-- tip
+9c5ce9b70771 tip
 -- glog
 @  11: r11 [] @ 11.00
 |