Why won't my zombie wander around the map?

Hi, I am trying to make my zombie wander around the map to different "destinations" until he sees a player and attacks. I keep getting this error in the output: " 16:10:49.207 - Workspace.FireZombie3.AI:50: attempt to perform arithmetic (sub) on number and nil"

Anything will help, thank you!

Here is my code:

function findDestination()
	local Map = game.Workspace.MapHolder:WaitForChild("Map")
	local destinationsfolder = Map:WaitForChild("Destinations")
	local destinations = destinationsfolder:GetChildren()
	for i = 1,#destinations do
		if (destinations[i].Position - myRoot.Position).Magnitude <= 25 then
			 oldDestination = destinations[i].Place.Value
			print(oldDestination)
		end
	end
	for i = 1,#destinations do
		if (destinations[i].Place.Value - oldDestination) == 1 then   -- This seems to be where the error occurs
		local goal = destinations[i].Position
		print(goal)
		return goal
		end
	end
end

function walkRandomly()
	local goal = findDestination()
	local path = game:GetService("PathfindingService"):CreatePath()
	path:ComputeAsync(myRoot.Position, goal)
	local waypoints = path:GetWaypoints()
	if path.Status == Enum.PathStatus.Success then
		for _, waypoint in ipairs(waypoints) do
			if waypoint.Action == Enum.PathWaypointAction.Jump then
				myHuman.Jump = true
			end
			myHuman:MoveTo(waypoint.Position)
			local timeOut = myHuman.MoveToFinished:Wait(1)
			if not timeOut then
				print("Got stuck")
				myHuman.Jump = true
				walkRandomly()
			end
		end
	else
		print("Path failed")
		wait(1)
		walkRandomly()
	end
end