orleans: [Question] Unexpected error serializing message - Collection was modified

Can you guys help me figure it out why I’m getting exceptions below. It seems that when I push (or consume?) message to the stream, serialization fails because dictionary was modified?

Runtime.Messaging.GatewaySender/GatewaySiloSender_1 100.104.2.49:11111] Unexpected error serializing message Request S100.104.2.49:30000:0*grn/5B91ACDA/00000000+WDC:87a3d5d3-d4e1-62b1-a05e-11bf2de91f72@15a52896->*cli/467c4d8e@eb8000bcd70c748e #7382800: global::Orleans.Streams.IStreamConsumerExtension:DeliverImmutable() on the gateway

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
   at Orleans.Serialization.BuiltInTypes.SerializeDictionary[K,V](Object original, BinaryTokenStreamWriter stream, Type expected)
   at Orleans.Serialization.SerializationManager.SerializeInner(Object obj, BinaryTokenStreamWriter stream, Type expected)
   at Orleankka.OrleansCodeGenNomnio_Unullo_Core_Contracts_MessageEnvelopeSerializer`1.Serializer(Object untypedInput, BinaryTokenStreamWriter stream, Type expected)
   at Orleans.Serialization.SerializationManager.SerializeInner(Object obj, BinaryTokenStreamWriter stream, Type expected)
   at Orleans.Serialization.BuiltInTypes.SerializeStringObjectDictionary(Object original, BinaryTokenStreamWriter stream, Type expected)
   at Orleans.Serialization.SerializationManager.SerializeMessageHeaderDictHelper(Dictionary`2 headers, BinaryTokenStreamWriter stream)
   at Orleans.Serialization.SerializationManager.SerializeMessageHeaders(Dictionary`2 headers, BinaryTokenStreamWriter stream)
   at Orleans.Runtime.Message.Serialize_Impl(Int32& headerLengthOut, Int32& bodyLengthOut)
   at Orleans.Runtime.Messaging.Gateway.GatewaySender.Send(Message msg, Socket sock)