FontForge freezes in generating 'aalt'

mauro sacchetto

FontForge would create 'aalt' "automagically"; so in its documentation.
Case 1:
1. I create a New lookup (Elements -- Information -- Lookup) in GSUB.
2. I choose Type: Single Substitution, and after:
3. Feature -- New : aalt.
4.1 With the right button of the mouse in the drop down menu now I choose:
Add 'aalt' feature. And FF freezes.
Case 2:
1., 2., 3.,
4.2 but this time I create the relative subtable with Add subtable.
With the new subtable I try 4,1. And FF freezes. Is this one a FF problem, or is there anything wrong in my method? Thank you