diff --git a/Bson/DefaultSerializer/Conventions/DiscriminatorConventions.cs b/Bson/DefaultSerializer/Conventions/DiscriminatorConventions.cs
index 3b1e0f7117..b5344cd27f 100644
--- a/Bson/DefaultSerializer/Conventions/DiscriminatorConventions.cs
+++ b/Bson/DefaultSerializer/Conventions/DiscriminatorConventions.cs
@@ -68,34 +68,38 @@ namespace MongoDB.Bson.DefaultSerializer.Conventions {
             BsonReader bsonReader,
             Type nominalType
         ) {
-            var bsonType = bsonReader.CurrentBsonType;
-
-            Type primitiveType = null;
-            switch (bsonType) {
-                case BsonType.Boolean: primitiveType = typeof(bool); break;
-                case BsonType.Binary:
-                    var bookmark = bsonReader.GetBookmark();
-                    byte[] bytes;
-                    BsonBinarySubType subType;
-                    bsonReader.ReadBinaryData(out bytes, out subType);
-                    if (subType == BsonBinarySubType.Uuid && bytes.Length == 16) {
-                        primitiveType = typeof(Guid);
-                    }
-                    bsonReader.ReturnToBookmark(bookmark);
-                    break;
-                case BsonType.DateTime: primitiveType = typeof(DateTime); break;
-                case BsonType.Double: primitiveType = typeof(double); break;
-                case BsonType.Int32: primitiveType = typeof(int); break;
-                case BsonType.Int64: primitiveType = typeof(long); break;
-                case BsonType.ObjectId: primitiveType = typeof(ObjectId); break;
-                case BsonType.String: primitiveType = typeof(string); break;
-            }
+            if (bsonReader.ReadState == BsonReadState.Value) {
+                Type primitiveType = null;
+                switch (bsonReader.CurrentBsonType) {
+                    case BsonType.Boolean: primitiveType = typeof(bool); break;
+                    case BsonType.Binary:
+                        var bookmark = bsonReader.GetBookmark();
+                        byte[] bytes;
+                        BsonBinarySubType subType;
+                        bsonReader.ReadBinaryData(out bytes, out subType);
+                        if (subType == BsonBinarySubType.Uuid && bytes.Length == 16) {
+                            primitiveType = typeof(Guid);
+                        }
+                        bsonReader.ReturnToBookmark(bookmark);
+                        break;
+                    case BsonType.DateTime: primitiveType = typeof(DateTime); break;
+                    case BsonType.Double: primitiveType = typeof(double); break;
+                    case BsonType.Int32: primitiveType = typeof(int); break;
+                    case BsonType.Int64: primitiveType = typeof(long); break;
+                    case BsonType.ObjectId: primitiveType = typeof(ObjectId); break;
+                    case BsonType.String: primitiveType = typeof(string); break;
+                }
 
-            if (primitiveType != null && nominalType.IsAssignableFrom(primitiveType)) {
-                return primitiveType;
+                if (primitiveType != null && nominalType.IsAssignableFrom(primitiveType)) {
+                    return primitiveType;
+                }
             }
 
-            if (bsonType == BsonType.Document) {
+            if (
+                bsonReader.ReadState == BsonReadState.Initial ||
+                bsonReader.ReadState == BsonReadState.Done ||
+                (bsonReader.ReadState == BsonReadState.Value && bsonReader.CurrentBsonType == BsonType.Document)
+            ) {
                 var bookmark = bsonReader.GetBookmark();
                 bsonReader.ReadStartDocument();
                 var actualType = nominalType;
diff --git a/DriverOnlineTests/DriverOnlineTests.csproj b/DriverOnlineTests/DriverOnlineTests.csproj
index 39f86e5193..98e892c5c3 100644
--- a/DriverOnlineTests/DriverOnlineTests.csproj
+++ b/DriverOnlineTests/DriverOnlineTests.csproj
@@ -76,6 +76,7 @@
     <Compile Include="Jira\CSharp77Tests.cs" />
     <Compile Include="Jira\CSharp92Tests.cs" />
     <Compile Include="Jira\CSharp93Tests.cs" />
+    <Compile Include="Jira\CSharp98Tests.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Samples\MagicDiscriminatorTests.cs" />
   </ItemGroup>
diff --git a/DriverOnlineTests/Jira/CSharp98Tests.cs b/DriverOnlineTests/Jira/CSharp98Tests.cs
new file mode 100644
index 0000000000..3c184de9a3
--- /dev/null
+++ b/DriverOnlineTests/Jira/CSharp98Tests.cs
@@ -0,0 +1,56 @@
+/* Copyright 2010 10gen Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using NUnit.Framework;
+
+using MongoDB.Bson;
+using MongoDB.Bson.DefaultSerializer;
+using MongoDB.Driver;
+using MongoDB.Driver.Builders;
+
+namespace MongoDB.DriverOnlineTests.Jira.CSharp98 {
+    [TestFixture]
+    public class CSharp98Tests {
+        private class A {
+            [BsonId]
+            public ObjectId Id { get; set; }
+            public int PA { get; set; }
+        }
+
+        private class B : A {
+            public int PB { get; set; }
+        }
+
+        [Test]
+        public void TestDeserializationOfTwoBs() {
+            var server = MongoServer.Create("mongodb://localhost/?safe=true");
+            var database = server["onlinetests"];
+            var collection = database.GetCollection<A>("csharp98");
+
+            collection.RemoveAll();
+            var b1 = new B { PA = 1, PB = 2 };
+            var b2 = new B { PA = 3, PB = 4 };
+            collection.Insert<A>(b1);
+            collection.Insert<A>(b2);
+
+            var docs = collection.FindAll().ToList();
+        }
+    }
+}