diff --git a/file_system.go b/file_system.go
index 33f7a79..95b7e51 100644
--- a/file_system.go
+++ b/file_system.go
@@ -114,9 +114,6 @@ type FileSystem interface {
 	// The file system is responsible for checking that the directory is empty.
 	//
 	// Sample implementation in ext2: ext2_rmdir (http://goo.gl/B9QmFf)
-	//
-	// TODO(jacobsa): Add tests for the assertion about directory link counts
-	// above (on a real file system and on memfs).
 	RmDir(
 		ctx context.Context,
 		req *RmDirRequest) (*RmDirResponse, error)
diff --git a/samples/memfs/posix_test.go b/samples/memfs/posix_test.go
index 4edb824..4224ced 100644
--- a/samples/memfs/posix_test.go
+++ b/samples/memfs/posix_test.go
@@ -24,6 +24,7 @@ import (
 	"path"
 	"testing"
 
+	. "github.com/jacobsa/oglematchers"
 	. "github.com/jacobsa/ogletest"
 )
 
@@ -297,3 +298,18 @@ func (t *PosixTest) ReadsPastEndOfFile() {
 	ExpectEq(0, n)
 	ExpectEq("", string(buf[:n]))
 }
+
+func (t *PosixTest) HardLinkDirectory() {
+	dirName := path.Join(t.dir, "dir")
+
+	// Create a directory.
+	err := os.Mkdir(dirName, 0700)
+	AssertEq(nil, err)
+
+	// Attempt to hard-link it to a new name.
+	err = os.Link(dirName, path.Join(t.dir, "other"))
+
+	AssertNe(nil, err)
+	ExpectThat(err, Error(HasSubstr("link")))
+	ExpectThat(err, Error(HasSubstr("not permitted")))
+}