validator is not a function error

Hi,
I'm not sure if this is a bug or an issue on my end, so I'm crossposting my stackoverflow post here - https://stackoverflow.com/questions/48520222/how-to-add-array-field-in-final-form

In short, it looks like the FieldArray field is returning a different function (versus a normal field), which is causing the runFieldLevelValidation function to cause an error.

Thanks in advance!