Raster > HSB adjust: -200 saturation colorizes turquoise

Steps to reproduce:

• open Inkscape
• draw a red rectangle
• make a bitmap copy
• Extensions > Raster > HSB Adjust: Saturation -200 (minimum value)

What happened?

The image turns turquoise.

Peek_2019-10-09_22-12

What should have happened?

The image turns grey.

Inkscape Version and Operating System:

• Inkscape Version: 1.0beta1 (ad9af741, 2019-10-09)
• Operating System: Linux Mint 19.2