Recuperar varbinary(MAX) do SQL Server para byte[] em C#

private static byte[] getDocument(int documentId)
{
    using (SqlConnection cn = new SqlConnection("..."))
    using (SqlCommand cm = cn.CreateCommand())
    {
        cm.CommandText = @"
            SELECT DocumentData
            FROM   Document
            WHERE  DocumentId = @Id";
        cm.Parameters.AddWithValue("@Id", documentId);
        cn.Open();
        return cm.ExecuteScalar() as byte[];
    }
}