In PostgreSQL the ASCII NUL or empty string \0 is seen as NOT NULL. This is because postgres developers consider any characters data. I know there are better discussions on it but I can’t find them right now. Unfortunately the programming language you are using probably doesn’t see it the same way. There’s a good chance that initialized variables are set to \0 and so when you try to insert from your language to a NOT NULL field with variables that are seen as undefined in your language, postgres accepts it, and now you have fields that you probably consider to have no data but are NOT NULL.