Steps to reproduce:

1. Make a csv and insert with values

> touch  ~/local/1/data/extern/test.csv
> echo "'adidas', NULL" > ~/local/1/data/extern/test.csv

2. Import the data from the local csv to the table

> CREATE TABLE test(a string, x bool);
> IMPORT INTO test(a, x) CSV DATA ("nodelocal://1/test.csv") WITH nullif="NULL";

ERROR: nodelocal://1/test.csv: error parsing row 1: parse "x" as BOOL: could not parse "NULL" as type bool: invalid bool value (row: 'adidas', NULL)

The reason is that NULL cannot be parsed due to the whitespace. Adding a hint would make this more obvious and likely save a lot of time.

The solution here would be to specify nullif=' NULL'

Jira issue: CRDB-17225
Epic CRDB-17785