Can't record - Error

[pstrg]

I'm trying to use WavePad to record audio from the Internet with no success.

In Options > Recording I've set:
- Device: Stereo Mix (Realtek(R) Audio)
- Input: Windows Record Mixer
However, when I hit F5 I'm always getting an error window:
Recorder Error
The recording failed to start. Please check your sound
devices are connected to your computer and turned on.

Note that I'm able to record using Audacity with Windows WASAPI as input (this option doesn't appear with WavePad).
OS is Windows 10 64-bit.