Getting access denied with GetEffectiveRightsFromAcl()..?

private enum TRUSTEE_FORM
    {
        TRUSTEE_IS_SID,
        TRUSTEE_IS_NAME,
        TRUSTEE_BAD_FORM,
        TRUSTEE_IS_OBJECTS_AND_SID,
        TRUSTEE_IS_OBJECTS_AND_NAME
    }

    private enum TRUSTEE_TYPE
    {
        TRUSTEE_IS_UNKNOWN,
        TRUSTEE_IS_USER,
        TRUSTEE_IS_GROUP,
        TRUSTEE_IS_DOMAIN,
        TRUSTEE_IS_ALIAS,
        TRUSTEE_IS_WELL_KNOWN_GROUP,
        TRUSTEE_IS_DELETED,
        TRUSTEE_IS_INVALID,
        TRUSTEE_IS_COMPUTER
    }

    private struct TRUSTEE
    {
        public IntPtr pMultipleTrustee;
        public MULTIPLE_TRUSTEE_OPERATION MultipleTrusteeOperation;
        public TRUSTEE_FORM TrusteeForm;
        public TRUSTEE_TYPE TrusteeType;
        public IntPtr ptstrName;
    }

    [DllImport("advapi32.dll", SetLastError = true)]
    private static extern void BuildTrusteeWithSid(
        ref TRUSTEE pTrustee,
        byte[] sid
    );

    [DllImport("advapi32.dll")]
    private static extern uint GetEffectiveRightsFromAcl(byte[] pacl, ref TRUSTEE
    pTrustee, ref uint pAccessRights);

    public bool HasAccess(SecurityIdentifier sid)
    {

        DiscretionaryAcl dacl = <DACL from somewhere>;

        byte[] daclBuffer = new byte[dacl.BinaryLength];
        dacl.GetBinaryForm(daclBuffer, 0);

        byte[] sidBuffer = new byte[sid.BinaryLength];
        sid.GetBinaryForm(sidBuffer, 0);

        TRUSTEE t = new TRUSTEE();
        BuildTrusteeWithSid(ref t, sidBuffer);

        uint access = 0;
        uint hr = GetEffectiveRightsFromAcl(daclBuffer, ref t, ref access);

        int i = Marshal.Release(t.ptstrName);

        return ((access | <Desired Access>) == <Desired Access>) ? true : false;
    }
}