# Troubleshooting
## "validate check digit failed"
```
Error invalid line 2. Error: validate check digit failed. NHS Number: 999 123 4561
```
The input parsed as ten digits but the tenth digit is not the correct
Modulus 11 check digit for the first nine digits. Possible causes:
* A typo in the input. Re-enter the number.
* The wrong column is being piped in. Double-check your `cut`/`awk`
command.
* The source system stores a non-NHS identifier that happens to be ten
digits long.
Recalculate the check digit by hand with the algorithm in
[About NHS Numbers](./about-nhs-numbers.md#check-digit) or use another
tool to cross-check.
## "Error parsing line"
```
Error parsing line 3. Error: ParseError. Line: not-an-nhs-number
```
The input does not contain ten digits (ignoring whitespace). Common
causes:
* Extra columns (e.g. passing a whole CSV row instead of a single field).
* Line endings included (rare, but possible with exotic encodings).
* A header row passed by mistake. Use `tail -n +2` to skip it.
* A comment or empty marker character such as `-` or `N/A`.
## CRLF line endings
On Windows, files often have `\r\n` line endings. Rust's
`BufRead::lines()` strips both `\r` and `\n`, so a CRLF file generally
pipes through cleanly. If you see unexpected parse errors against a
Windows-authored file, strip the carriage returns explicitly and try
again:
```sh
tr -d '\r' < input.txt | nhs-number-cli
```
A more common Windows issue is **trailing whitespace** (a stray space
or tab at the end of a line). That is not stripped by the reader, so
the parser rejects the line. Normalise first:
```sh
sed 's/[[:space:]]*$//' < input.txt | nhs-number-cli
```
## No output at all
* Did you forget to pipe anything in? The tool reads from `stdin`; if
`stdin` is empty, the tool exits immediately with no output.
* Did you redirect stderr somewhere you forgot? Try running without
redirection first to see all diagnostics.
## The binary runs but exits with status 0 despite invalid numbers
This is by design. See [Usage ยง exit status](./usage.md#exit-status). If
you need a non-zero exit on error, wrap the call in a script that
inspects the `stderr` stream.
## `cargo build` fails with "edition = 2024" error
You are on an older toolchain. Upgrade:
```sh
rustup update stable
```
Edition 2024 requires Rust 1.85 or newer.
## Reporting a bug
Open an issue at
<https://github.com/joelparkerhenderson/nhs-number-cli/issues>
and include:
* The command you ran.
* A small, **synthetic** input file that reproduces the problem. Do not
include real NHS Numbers.
* The complete `stdout` and `stderr` output.
* Output of `rustc --version` and `cargo --version`.