diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 65eb790fd0..610849a653 100644
--- a/builtin/merge-file.c
+++ b/builtin/merge-file.c
@@ -77,6 +77,7 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 					argv[i]);
 	}
 
+	xmp.ancestor = names[1];
 	xmp.file1 = names[0];
 	xmp.file2 = names[2];
 	ret = xdl_merge(mmfs + 1, mmfs + 0, mmfs + 2, &xmp, &result);
diff --git a/t/t6023-merge-file.sh b/t/t6023-merge-file.sh
index 5034dd1352..d486d73994 100755
--- a/t/t6023-merge-file.sh
+++ b/t/t6023-merge-file.sh
@@ -181,7 +181,7 @@ et nihil mihi deerit;
 
 In loco pascuae ibi me collocavit;
 super aquam refectionis educavit me.
-|||||||
+||||||| new5.txt
 et nihil mihi deerit.
 In loco pascuae ibi me collocavit,
 super aquam refectionis educavit me;
@@ -225,7 +225,7 @@ et nihil mihi deerit;
 
 In loco pascuae ibi me collocavit;
 super aquam refectionis educavit me.
-||||||||||
+|||||||||| new5.txt
 et nihil mihi deerit.
 In loco pascuae ibi me collocavit,
 super aquam refectionis educavit me;