Can not use Serial port in Unity 5.4

Serial Port is existed in ‘System.IO.Ports’.
It can be used when selecting ‘.Net 2.0’ instead of ‘.Net 2.0 Subset’.
I can have used Serial Port on Standalone platform in Unity 5.4 lower version.
But, it was occured exception when using Serial Port after upgrading Unity 5.4.
The message of exception is ‘System.DllNotFoundException: MonoPoxisHelper’.
I think that some DLL is missing when building new Unity.
This is very important problem, so I am really looking forward to resolve it.
I hope the quick action of Unity about this problem.

@SmartMaker
We have ran into the same issue. We can not connect to serial port in Unity 5.4, only earlier versions. Please report a bug and post the link here so my team and I can upvote it with you.

EDIT:
It is reported fixed in 5.4.1