• You have any Lookup for a field which is nullable in a Form with a connected Model (e.g. via hasOne reference)
• You have selected an entry of the Lookup and saved.
• You then select the empty entry of that Lookup and save again

Result: Atk4/Data tries to store 0, instead of NULL. This is wrong, it has to be NULL.
Sorry can't provide a quick demo, but this should be straightforward.
Funnily, in Dropdown control it works, only in Lookup not.