Cannot threshold composite float32 images

[kephale]

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:

[imagejan]

This is not restricted to 32-bit images. In my tests, every composite image was giving this Threshold dialog, e.g.:

run("Blobs (25K)");
run("Merge Channels...", "c1=blobs.gif c2=blobs.gif c3=blobs.gif create");
run("Threshold...");