Why does UnicastIPAddressInformation.IPv4Mask return null to IPv4 address?

Below is a code snippet that builds a list of IP addresses and their subnet masks from the local system, however the Warn function seems to fire regularly, which is theoretically impossible, since there shouldn't be an IPv4 address without a matching subnet mask [?].

    static NetworkUtil()
    {
        foreach (NetworkInterface ni in NetworkInterface.GetAllNetworkInterfaces())
        {
            foreach (UnicastIPAddressInformation address in ni.GetIPProperties().UnicastAddresses)
            {
                if (address.Address.AddressFamily == AddressFamily.InterNetwork)
                {
                    if (address.IPv4Mask != null)
                    {
                        m_subnets.Add(address.Address, address.IPv4Mask);
                    }
                    else
                    {
                        m_log.Warn("[NetworkUtil] Found IPv4 Address without Subnet Mask!?");
                    }
                }
            }
        }
    }