Digital Light & Color

I'm trying to create custom curves in the Stack Image transformation. Using a Q-13 card I have made three bracketed exposures (-2, 0, +2). I have cropped to where only the grey-scale is visible. I open the Stack Image transformation and load the underexposed image (-2) as Image 1, the overexposed image (+2) as Image 2, and the normal image (0) as Image 3. I select Build Curves: Q-13, accept default Black and White points, and Yes to include Normal image. When I select "OK", I get this message, "Images are in the wrong order. You must have the underexposed one first, the overexposed one next, and the normal image third." I have repeated this process over and over with the same result leading me closer and closer to the "definition of insanity". :-) I used aperture priority and the exposures are as follows in this order: underexposure (-2) 1/500s:F4.0, overexposure (+2) 1/30s:F4.0, and normal (0) 1/125s:F4.0. Has anyone experienced a similar problem, have a suggestion, or a solution?
Thanks!

First make sure the images have the lightest patches on the left and the darkest on the right. This may require using Transformation/Geometry/Mirror/Rotate.

The error you mentioned occurs if the darkest patch of the first image is brighter than the darkest patch of the normal image or the darkest patch of the normal image is brighter than the darkest patch of the overexposed image. This might happen if clipping is happening which might be caused by over- or under-exposing the normal image.

Thanks for your reply, Jonathan! I began with Sony .ARW files and converted to 16 bit .tiff using Bibble default values with no adjustments. I wanted to do as little processing as possible so the only PW transformations I used was Level and Crop. The images are oriented correctly with the light end to the left. The comparative brightness between the darkest patches is what one would expect to see. Checking the histograms reveals the exposure shifts you'd expect with -2, 0, +2, bracketing. However, what did the trick and resulted in Custom Curves being created, was expanding the dynamic range via the Levels and Color — Full Range transformation. After applying this transform to each image, I followed the same procedure with Stack Images as before, however this time, the Custom Curves were created without a hitch. I plan on doing some more work with High Dynamic Range (HDR) images and comparing PW and Photomatix. I'm not nearly as interested in having a product that works quickly (Photomatix) as I am with one that produces superior results. Thanks for a great product!