Cannot threshold composite float32 images

To reproduce:
1) Open blobs sample image
2) Convert blobs to float32
3) In GUI: Image > Color > Merge Channels
4) Add blobs to 2 channels
5) Try to threshold resulting composite image

Observed behavior:

Reproduce the failure with the blobs sample image: convert it to float32, use Image > Color > Merge Channels, add blobs to two channels, and threshold the composite image. Start by tracing the GUI actions for channel merging and thresholding. Done means the composite float32 image can be thresholded without the observed failure.

Written by the indexing model from the issue text.