The registered image is black when the fixed image is very small

[zerglog]

Dear Author:
I tried to use the elastix to register MRI brain image. The size of fixed image I used is about 27MB and moving is about 55MB. I used the default rigid registration parameter in the elastix website(only change I made is to change the demension of image from 2 to 3). Is there any requirement about the inpuy image? I tried the larger fixed image the moving image about 500MB. It is totally fine. But when I use the small size image, the result is black.