Unity3D + protobuf-net: Could not load type of field Protobuf.BufferPool:_pool due to... System.Buffers

I'm trying to use protobuf-net in Unity but I get the following exception when I try:

TypeLoadException: Could not load type of field 'ProtoBuf.BufferPool:_pool' (0) due to: Could not load file or assembly 'System.Buffers, Version=4.0.2.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. assembly:System.Buffers, Version=4.0.2.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51 type:<unknown type> member:(null) signature:<none>
ProtoBuf.ProtoWriter+StreamProtoWriter.CreateStreamProtoWriter (System.IO.Stream dest, ProtoBuf.Meta.TypeModel model, System.Object userState) (at <7f2f6774d3104e98a2359d5e5462b6bc>:0)
ProtoBuf.ProtoWriter+State.Create (System.IO.Stream dest, ProtoBuf.Meta.TypeModel model, System.Object userState) (at <7f2f6774d3104e98a2359d5e5462b6bc>:0)
ProtoBuf.Serializer.Serialize[T] (System.IO.Stream destination, T instance, System.Object userState) (at <bd1d3a3f27ab42199d646f1541550b36>:0)
ProtoBuf.Serializer.Serialize[T] (System.IO.Stream destination, T instance) (at <bd1d3a3f27ab42199d646f1541550b36>:0)
ProtobufNetTest.Awake () (at Assets/Tests/Protobuf-net/ProtobufNetTest.cs:26)

I've installed protobuf-net using nuget for unity. The test script I'm using looks like this:

using ProtoBuf;
using System.IO;
using UnityEngine;


[ProtoContract]
public class ProtobufTestItself
{
    [ProtoMember( 1 )]
    public int IntMember { get; set; }

}

public class ProtobufNetTest : MonoBehaviour
{
    void Awake()
    {
        var test = new ProtobufTestItself();

        test.IntMember = 313373;

        byte[] bytes = null;

        using ( var memoryStream = new MemoryStream() )
        {
            Serializer.Serialize( memoryStream, test );

            bytes = memoryStream.ToArray();
        }

        using ( var memoryStream = new MemoryStream( bytes ) )
        {
            var roundTripped = Serializer.Deserialize<ProtobufTestItself>( memoryStream );

            Debug.Log( "Round tripped: " + roundTripped.IntMember.ToString() );
        }
    }
}

And this is using Unity 2019.4.15f1.

How do I go about fixing this?