substring_error negative substring length not allowed — 22011

Symptoms

A substring()/substr() call was given a negative length. Length must be non-negative, so PostgreSQL raises SQLSTATE 22011 (substring_error).

• The length argument to substring is negative.
• Common with computed lengths (e.g. position arithmetic).
• Negative length has no defined meaning.

What the server log shows

ERROR:  negative substring length not allowed

Why PostgreSQL raises this — what the manual says

As Section 9.4 String Functions and Operators explains:

The count (length) argument to substring() must not be negative; a negative length has no meaning and is rejected — pass zero or a positive length, or omit the FOR clause to return the rest of the string.

substring reads length characters from the start position. A negative length is meaningless (you cannot read a negative number of characters), so PostgreSQL rejects it with 22011.

Common causes

• A computed length that became negative (e.g. end - start with end < start).
• Off-by-one or sign errors in length arithmetic.
• Unvalidated length input.

How to fix it

1. Clamp the length to be non-negative (e.g. GREATEST(len, 0)).
2. Fix the arithmetic so length is never negative.
3. Validate position/length inputs before calling substring.

Related & next steps

• SQLSTATE 22011 — Substring Error (full reference)
• Message: invalid input syntax for type bigint
• Glossary: String functions

Reference: PostgreSQL 18 Section 9.4 “String Functions”.