got FunctionClauseError while inspecting

[garthk]

import Norm
schema(%Date{
  day: spec(is_integer() and & &1 > 0),
  month: spec(is_integer() and & &1 > 0),
  year: spec(is_integer() and & &1 > 0)
})

→

#Norm.Schema<%Inspect.Error{
  message: "got FunctionClauseError with message \"no function clause matching in Calendar.ISO.date_to_string/4\" while inspecting %{__struct__: Date, calendar: Calendar.ISO, day: #Norm.Spec<is_integer() and &(&1 > 0)>, month: #Norm.Spec<is_integer() and &(&1 > 0)>, year: #Norm.Spec<is_integer() and &(&1 > 0)>}"
}>