VerificationException when using Webplayer after updating to Unity3

"VerificationException: Error verifying ProtoBuf.Serializer`1:Deserialize (T&,ProtoBuf.SerializationContext): Token and source types of stobj don't match at 0x01f6"

We get the above error when running our game with Web Player deployment selected. Obviously we are using Protobuf-net. We are pretty new to Unity, so I'm sure we are doing something wrong. What we know is that it works fine in standalone and it worked with the Web Player before updating to 3.0. We know about the new security protocol and have fixed it so we can get past the auth and connect to our server just fine, just can't use Protobuf. This is the line of code it errors on:

PacketServer packet = Serializer.DeserializeWithLengthPrefix<PacketServer>(ms, PrefixStyle.Base128);

Any advice/ideas? Thanks.

We tried some different builds of protobuf-net and found that the "NET20" build does work. We were using their "MONO" build before.