BUG REPORT

```csharp
var converter = new BalancedVariantConverter(links);
var name_root = converter.Convert("super_name".ToList().Select((c) => (ulong)c).ToList());
var notation = links.FormatStructure(name_root, _ => false, true, true);
Console.WriteLine(notation);
```
This code throw exception

Start by running the provided C# example with BalancedVariantConverter and links.FormatStructure, recording the exception details that are currently missing. Trace the failure through those entry points and establish a focused regression test; the issue is done when the reported example no longer throws or the expected failure is clearly documented.

Written by the indexing model from the issue text.