diff --git a/builtin-read-tree.c b/builtin-read-tree.c
index 793eae0a5f..5fb84b81a7 100644
--- a/builtin-read-tree.c
+++ b/builtin-read-tree.c
@@ -228,6 +228,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		if (0 <= pos)
 			die("file '%.*s' already exists.",
 					pfxlen-1, opts.prefix);
+		opts.pos = -1 - pos;
 	}
 
 	if (opts.merge) {