All errors › ORA errors
ORA-01843: Not a valid month
What it means
The month part of the input does not match the format: wrong mask, wrong NLS_DATE_LANGUAGE for month names, or day and month swapped.
How to fix it
Always pass an explicit mask, and for month names also the language: TO_DATE(s, 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE=ENGLISH').
Catch it before it runs
ORA Error Helper
Explains ORA-/PLS- codes inside JetBrains IDEs and DataGrip. PL/SQL Lint
Flags risky PL/SQL patterns while you type. Oracle Plan Reader
Reads execution plans and points at the slow step. PL/SQL in Practice
Kindle book with practical PL/SQL for working developers.
Explains ORA-/PLS- codes inside JetBrains IDEs and DataGrip. PL/SQL Lint
Flags risky PL/SQL patterns while you type. Oracle Plan Reader
Reads execution plans and points at the slow step. PL/SQL in Practice
Kindle book with practical PL/SQL for working developers.
Nearby errors
Updated 2026-11-12.