Unity Networking Serialize Array of Vector3 and Quaternion

i'm trying to serialize this structs for sending it via Unity NetworkList. Probably i'm doing the wrong serialization but idk what, unity documentation: https://docs-multiplayer.unity3d.com/docs/advanced-topics/serialization/inetworkserializable

public struct PlayerPositionData : INetworkSerializable, IEquatable<PlayerPositionData>
{
    public int numeroAncora;
    public Vector3[] posizioni;
    public Quaternion[] rotazioni;
 
    public PlayerPositionData(int numeroAnc, Vector3[] posiz, Quaternion[] rotaz)
    {
        numeroAncora = numeroAnc;
        posizioni = posiz;
        rotazioni = rotaz;
    }
 
 
    public void NetworkSerialize<T>(BufferSerializer<T> serializer) where T : IReaderWriter
    {
        serializer.SerializeValue(ref numeroAncora);
 
        //SERIALIZZO ARRAY POSIZIONI
        int length = 0;
        if (!serializer.IsReader)
        {
            length = posizioni.Length;
        }
 
        serializer.SerializeValue(ref length);
 
        // Array
        if (serializer.IsReader)
        {
            posizioni = new Vector3[length];
        }
 
        for (int n = 0; n < length; ++n)
        {
            serializer.SerializeValue(ref posizioni[n]);
        }
 
 
     
        //SERIALIZZO ARRAY ROTAZIONI
        if (!serializer.IsReader)
        {
            length = rotazioni.Length;
        }
 
        serializer.SerializeValue(ref length);
 
        // Array
        if (serializer.IsReader)
        {
            rotazioni = new Quaternion[length];
        }
 
        for (int n = 0; n < length; ++n)
        {
            serializer.SerializeValue(ref rotazioni[n]);
        }
    }
 
    public bool Equals(PlayerPositionData other)
    {
        return posizioni.Length == other.posizioni.Length && numeroAncora == other.numeroAncora;
    }
}

and this:

public struct PlayerData : INetworkSerializable, IEquatable<PlayerData>
{
    public ulong idRete;
    public int numeroPlayer;
    public PlayerPositionData posizioni;
 
    public PlayerData(ulong idRet, int payerN, PlayerPositionData posiz)
    {
        idRete = idRet;
        numeroPlayer = payerN;
        posizioni = posiz;
    }
 
 
    public void NetworkSerialize<T>(BufferSerializer<T> serializer) where T : IReaderWriter
    {
        serializer.SerializeValue(ref idRete);
        serializer.SerializeValue(ref numeroPlayer);
 
        posizioni.NetworkSerialize(serializer);
    }
 
    public bool Equals(PlayerData other)
    {
        return idRete == other.idRete;
    }
}

in this way:

private NetworkList<PlayerData> datiPlayers = new NetworkList<PlayerData>();

but i recive this error:

error CS8377: The type 'PlayerData' must be a non-nullable value type, along with all fields at any level of nesting, in order to use it as parameter 'T' in the generic type or method 'NetworkList<T>'

why? the PlayerData struct work without nested PlayerPositionData...