Update fluent validation library
There are bug-fixes and improvements made to the original fluent validation source at:

Fluent Validation has been updated to the latest release in v4.58:
http://docs.servicestack.net/releases/v4.5.8#fluent-validation-upgraded
Which also includes built-in support for Async Validators
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wwwlicious commented
FluentValidation has been updated to 6.2.1 (the latest at the time of this post) as part of this pull request
https://github.com/ServiceStack/ServiceStack/pull/1123
It is available on the myget feed and will be in the next release.
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Drazen Dotlic commented
I think the better thing would be to just add dependency on FluentValidation and stop "including" it in the framework, or just ignore it and let us add a dependency ourselves.