Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
Chances are your TIFF delegate library does not have LZW compression enabled and ImageMagick is failing to report a warning. If you use compression none does it work? We tried your command with ImageMagick 6.2.7, the current release, and it returned a proper LZW-compressed TIFF image.