diff --git a/samples/flushfs/flush_fs_test.go b/samples/flushfs/flush_fs_test.go
index 2df75ad..5b232ab 100644
--- a/samples/flushfs/flush_fs_test.go
+++ b/samples/flushfs/flush_fs_test.go
@@ -1003,7 +1003,8 @@ func (t *ReadOnlyTest) CreateFile() {
 }
 
 func (t *ReadOnlyTest) Mkdir() {
-	AssertTrue(false, "TODO")
+	err := os.Mkdir(path.Join(t.Dir, "blah"), 0700)
+	ExpectThat(err, Error(HasSubstr("read-only")))
 }
 
 func (t *ReadOnlyTest) OpenForWrite() {