I am using QGIS 3.4 and when I finish drawing a feature (line), I try to add a negative value (-1) in the attribute dialog and it is automatically changed to NULL. The field is integer.
Even if I leave it NULL and then open the attribute table and try to edit it there, it still does not let me use a negative number - automatically changes to NULL. The only way is to change it using Field Calculator and update the existing field to -1.
I tried the same in version 2.18.17 and it works fine.
I haven't used QGIS for years and I am not entirely familiar with the changes in the latest versions. Is there a new setting/option that I don't know of?
I have reported the bug and it has now been fixed.
The new QGIS versions won't have this issue