How do I use @ConfigItem in Quarkus Extension

Question

@ConfigGroup
public static class Properties {

 @ConfigItem  
 public List<Server> servers = new ArrayList<>();
}

When I build projects

Errors:

Runtime java.lang.IllegalArgumentException: SRCFG00013: No Converter registered for class io.quarkiverse.xx.xx.cfg.xxx$Server

but build succeeds when I don't use the list

If you know the solution or the related documents, please let me know, thank you