--- a/tests/test-diffdir
+++ b/tests/test-diffdir
@@ -1,6 +1,7 @@
 #!/bin/sh
 
 hg init
+hg branch dummy # needed so -r "" doesn't point to the unnamed/default branch
 touch a
 hg add a
 hg ci -m "a" -d "1000000 0"