Substitutions with writef() or format()?

[Ron Tarrant]

'Morning, all.

If these lines:

    string currency = format("$%,.02f", 11_234_456.99);
    writeln(currency);

Will result in:

    $11,234,456.99

Why don't these lines:

    string notCurrency = format("%,", 11_234_456);
    writeln(notCurrency);

result in:

    11,234,456

???

Instead of a range violation?