Blend Mask Question

[tmr4]

I've been experimenting with the Blend Mask tool on the horsehead.psd example file (pg 104+ in Hidden Power 3). I tried to expand the example by creating masks for the shadows, midtones and highlights. With all three masks created it seems as if they would combine to create the original picture. However, with the background layer turned off several areas of the resulting picture are transparent. Is the Blend Mask tool missing some detail in the process? I've been very careful to ensure that the three masks cover a continuous portion of the curve range (i.e., 0 to 25%, 26 to 75% and 76 to 100%, all at 100%). I also played with the options settings on the Curves dialog (setting the clipping to zero and selecting different other options) with no success. I guess I'm missing something.

Any ideas?