0.93.1: "Image->Count colors used" always returns 2 for binary images

[TornadoOfSouls]

XnView: MP 0.93.1 - 64-bit
OS: Windows 7 - 64-bit

To reproduce:
1. Open solid color binary (1 bpp) image
2. Press Image->Count colors used, XnView returns "2"
3. Press Image->Change color depth... and select Grayscale, Colors or 24/32 bits
4. Press Image->Count colors used, XnView returns "1"

Expected behaviour: Get the actual number of used colors.