Indigo facilitates serial communication through a Microsoft Serial Communication library that limits com ports to port 16 and below. If possible, reassign your serial port to com port 16 or below.

Here is a help document on how to change the COM port settings:
Change Com Port Assignment in Windows

If you cannot change the port number for your serial device, then there is an alternate solution. You will need to replace the Microsoft Serial Communication library with a modified version that enables ports above 16. Please download the following ZIP file and read the instructions for more information.
Alternate Serial Communications Library