in case it could help here's the stack trace:

Code:
System.Runtime.Fx.AllocateByteArray(Int32 size)
System.Runtime.InternalBufferManager.PooledBufferManager.TakeBuffer(Int32 bufferSize)
System.Runtime.BufferedOutputStream.AllocNextChunk(Int32 minimumChunkSize)
System.Runtime.BufferedOutputStream.WriteCore(Byte[] buffer, Int32 offset, Int32 size)
System.Runtime.BufferedOutputStream.Write(Byte[] buffer, Int32 offset, Int32 size)
System.Xml.XmlStreamNodeWriter.FlushBuffer()
System.Xml.XmlUTF8NodeWriter.WriteBase64Text(Byte[] buffer, Int32 offset, Int32 count)
System.Xml.XmlUTF8NodeWriter.WriteBase64Text(Byte[] trailBytes, Int32 trailByteCount, Byte[] buffer, Int32 offset, Int32 count)
System.Xml.XmlBaseWriter.WriteBase64(Byte[] buffer, Int32 offset, Int32 count)
System.Xml.XmlDictionaryWriter.WriteValue(IStreamProvider value)
System.ServiceModel.Dispatcher.StreamFormatter.Serialize(XmlDictionaryWriter writer, Object[] parameters, Object returnValue)
System.ServiceModel.Dispatcher.OperationFormatter.SerializeBodyContents(XmlDictionaryWriter writer, MessageVersion version, Object[] parameters, Object returnValue, Boolean isRequest)
System.ServiceModel.Dispatcher.OperationFormatter.OperationFormatterMessage.OperationFormatterBodyWriter.OnWriteBodyContents(XmlDictionaryWriter writer)
System.ServiceModel.Channels.BodyWriter.WriteBodyContents(XmlDictionaryWriter writer)
System.ServiceModel.Channels.BodyWriterMessage.OnWriteBodyContents(XmlDictionaryWriter writer)
System.ServiceModel.Channels.Message.OnWriteMessage(XmlDictionaryWriter writer)
System.ServiceModel.Channels.Message.WriteMessage(XmlDictionaryWriter writer)
System.ServiceModel.Channels.BufferedMessageWriter.WriteMessage(Message message, BufferManager bufferManager, Int32 initialOffset, Int32 maxSizeQuota)
System.ServiceModel.Channels.TextMessageEncoderFactory.TextMessageEncoder.WriteMessage(Message message, Int32 maxMessageSize, BufferManager bufferManager, Int32 messageOffset)
System.ServiceModel.Channels.HttpOutput.SerializeBufferedMessage(Message message)
System.ServiceModel.Channels.HttpOutput.Send(TimeSpan timeout)
System.ServiceModel.Channels.HttpRequestContext.OnReply(Message message, TimeSpan timeout)
System.ServiceModel.Activation.HostedHttpContext.OnReply(Message message, TimeSpan timeout)
System.ServiceModel.Channels.RequestContextBase.Reply(Message message, TimeSpan timeout)
System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.Reply(MessageRpc& rpc)