HoloLens and raspberry pi serial communication

I’m using command SerialPort sp = new SerialPort(“COM6”, 9600); to access the port and i’m using MRTk deploying on HoloLens. but it gives me

“Assets\Data_Recording.cs(22,5): error CS0433: The type ‘SerialPort’ exists in both ‘System.IO.Ports, Version=4.0.1.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51’ and ‘System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089’”

This error and i’m using .NET Framework