Implicit operator causing sneaky NullReferenceException

ImplicitCausingNullRef.cs

void Main()
{
	var order = new Order
	{
		Instructions = SpecialInstructions.LeaveAtDoor,
	};
	
	var thing = new Thing
	{
		Instructions = order.Instructions,
	};
	
	thing.Dump();
}

// Define other methods and classes here
public class Order
{
	public SpecialInstructions Instructions {get; set;}
}
public class Thing
{
	public string Instructions { get; set; }
}

public class SpecialInstructions
{
	public static SpecialInstructions LeaveAtDoor = new SpecialInstructions(1, "Leave at door");
	
	private SpecialInstructions(int value, string displayName)
	{
		Value = value;
		DisplayName = displayName;
	}
	
	public int Value {get; private set;}
	public string DisplayName {get; private set;}
	
	public static implicit operator string(SpecialInstructions value)
	{
		return value.DisplayName;
	}
}