.NET 4.6 プロジェクトだとエラーになるけど、PCL だとエラーにならない構造体

Ufcpp.cs

using System;

namespace Ufcpp
{
    public struct Ufcpp
    {
        private readonly DateTimeOffset _dateTime;

        public Ufcpp(DateTimeOffset dateTime)
        {
            this._dateTime = dateTime;
        }
        
        public Ufcpp(DateTime dateTime)
        {
            // CS0171
            // Field 'Ufcpp._dateTime' must be fully assigned before control is returned to the caller
        }
    }
}