MyFonts online font validator may reject uploaded fonts with the following error message:

Here is the answer from MyFonts:

>Yes, we are aware that this is indeed present for some font files. Our validator is detecting that your fonts are missing glyph class definitions. This is an older error that used to be a problem but no longer prevents fonts from working in Macs. Our developers are going to be updating the validator to remove this check.

>As a short-term workaround, you can just add a GDEF table to each font. The missing information is glyph class definitions.

Well, indeed FontLab VI doesn't export GDEF table (which is not mandatory) if you haven't assigned classes to glyphs. But if you assign some glyph class to at least one glyph, for example, to "A" then FontLab will generate the GDEF table. You can easily do this in the Glyph panel:

and this will make MyFonts font validator happy.