This occurs on the latest git source as of today (8d1d29365ebefea40d2c).

[Test]
public void Test()
{
var table = new Hashtable();
table["Text"] = "hello";
table["Enum"] = E.B;
new Doc

.ToBson();
}

The above test will fail with a MongoDB.Bson.BsonSerializationException : EnumSerializer cannot be used with type: System.Object.