Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
In recent versions of ImageMagick we enhanced the fuzz factor to account for the alpha channel and in the process fixed a bug. So your command should work the same across ImageMagick versions but with a different fuzz value required for recent versions of ImageMagick.
Its a bug in the -opaque option. ImageMagick multiplies the fuzz associated with the fill and target colors. The target is 0 so the fuzz result is 0. Will fix this bug in ImageMagick 6.2.7 Beta scheduled for release in the next week or two.