TCP/IP lost ack packet - Githubissues

stack stalls waiting on FIN from other end:
(gdb) bt
#0  Microsoft.Zelig.Runtime.ThreadManager::SwitchToWait#3844 (this=<optimized out>, wr=<optimized out>) at ThreadManager.cs:300
#1  Microsoft.Zelig.Runtime.Synchronization.WaitingRecord::Wait#6170 (this=) at WaitingRecord.cs:214
#2  Microsoft.Zelig.Runtime.Synchronization.WaitingRecord.Holder::get_RequestProcessed#6931 (this=) at WaitingRecord.cs:108
#3  System.Threading.EventWaitHandle.EventWaitableObject::Acquire#6570 (this=<optimized out>, timeout=...) at EventWaitHandleImpl.cs:82
#4  System.Threading.WaitHandle::WaitOne#4782 (this=<optimized out>, timeout=...) at WaitHandleImpl.cs:76
#5  Microsoft.Zelig.LlilumOSAbstraction.CmsisRtos.CmsisRtosSemaphore::Acquire#21624 (this=<optimized out>, timeout=<optimized out>) at CmsisRtosSemaphore.cs:61
#6  Microsoft.Zelig.LlilumOSAbstraction.CmsisRtos.CmsisRtos::osSemaphoreWaitEx#21582 (semaphore_id=, millisec=<error reading variable: Cannot access memory at address 0xffffffff>)
at CmsisRtos.cs:503
#7  osSemaphoreWait (semaphore_id=<optimized out>, millisec=<optimized out>) at sys_arch.c:509
#8  sys_arch_sem_wait (sem=, timeout=timeout@entry=0x0) at sys_arch.c:283
#9  tcpip_apimsg (apimsg=apimsg@entry=) at tcpip.c:313
#10 netconn_delete (conn=) at api_lib.c:114
#11 lwip_close (s=<optimized out>) at sockets.c:459
#12 LLOS_lwip_close (socket=<optimized out>) at mbed_socket.cpp:83
#13 Microsoft.Llilum.Net.SocketNative::close#5168 () at SocketProvider.cs:220
#14 System.Net.Sockets.Socket::Dispose#15708 (this=<optimized out>) at Socket.cs:398
#15 Microsoft.Zelig.Test.mbed.SimpleNet.Program::Main#155 () at Program.cs:52
#16 Microsoft.Zelig.Runtime.ThreadManager::MainThread#3867 () at ThreadManager.cs:543
#17 System.Threading.ThreadStart::Invoke#9252 () at KernelList.cs:30
#18 System.Threading.Thread::Entrypoint#4859 (this=) at ThreadImpl.cs:230
NETMF / llilum

TCP/IP lost ack packet #198
