Python backtrace when opening a .tf file

I'm getting this backtrace when I try to open a terraform file .tf. This is on Win10. It should open VS Code as it does for other files, like .py files. The installation is bog standard, just installed fman.

Traceback (most recent call last):
File "fman\impl\plugins\command_registry.py", line 128, in execute_command
File "C:\Users\roy000\AppData\Local\fman\Versions\1.7.2\Plugins\Core\core\commands_init.py", line 294, in call
open_files([url], self.pane)
File "C:\Users\roy000\AppData\Local\fman\Versions\1.7.2\Plugins\Core\core\commands_init.py", line 327, in _open_files
open_local_files(local_file_paths, pane)
File "C:\Users\roy000\AppData\Local\fman\Versions\1.7.2\Plugins\Core\core\commands_init.py", line 331, in _open_local_files
open_local_files_win(paths, pane)
File "C:\Users\roy000\AppData\Local\fman\Versions\1.7.2\Plugins\Core\core\commands_init.py", line 362, in _open_local_files_win
ShellExecute(0, None, path, None, cwd, SW_SHOWNORMAL)
pywintypes.error: (31, 'ShellExecute', 'A device attached to the system is not functioning.')

Start in core/commands/init.py at _open_local_files_win and reproduce opening a .tf file on Windows 10, using the reported ShellExecute traceback as the starting point. Done means .tf files open in VS Code without the error, as .py files do.