diff --git a/Documentation/howto/using-merge-subtree.txt b/Documentation/howto/using-merge-subtree.txt
index 1ae8d1214e..a499a94ac2 100644
--- a/Documentation/howto/using-merge-subtree.txt
+++ b/Documentation/howto/using-merge-subtree.txt
@@ -33,7 +33,7 @@ Here is the command sequence you need:
 
 ----------------
 $ git remote add -f Bproject /path/to/B <1>
-$ git merge -s ours --no-commit Bproject/master <2>
+$ git merge -s ours --no-commit --allow-unrelated-histories Bproject/master <2>
 $ git read-tree --prefix=dir-B/ -u Bproject/master <3>
 $ git commit -m "Merge B project as our subdirectory" <4>