Help with detecting incoming projectiles in front of my character?

Could someone please help me with my script? I am uncertain as to why it is killing me despite the absence of a moving part heading in my direction. Any help would be greatly appreciated.

-- Get the position of the player character
local charPosition = game.Players.LocalPlayer.Character.HumanoidRootPart.Position

-- Create a box that encompasses 20 blocks in front of the character
local boxSize = Vector3.new(20, 20, 20)
local boxCFrame = CFrame.new(charPosition + Vector3.new(10, 0, 0), charPosition)

-- Use GetPartBoundsInBox to find any parts that are in the box
local parts = game.Workspace:GetPartBoundsInBox(boxCFrame, boxSize)

-- Check each part to see if it is moving and if it will hit the character's torso
for _, part in pairs(parts) do
	local partVelocity = part.Velocity
	local partPosition = part.Position
	local partToCharVector = charPosition - partPosition
	local partToCharDistance = partToCharVector.magnitude

	-- If the part is moving and it is closer than 20 blocks from the character,
	-- then it might hit the character's torso; destroy my Head
	if partVelocity.magnitude > 0 and partToCharDistance < 20 then
		if partVelocity:Dot(partToCharVector) > 0 then
			-- If the part is moving towards the character,destroy my characters head
			game.Players.LocalPlayer.Character.Head:Destroy()
		end
	end
end

The script only runs once. To repeat it, you should use a loop, more specifically a while loop.

local Players = game:GetService("Players")
local player = Players.LocalPlayer

while true do
	local character = player.Character
	if not character or not character.Parent then
		character = player.CharacterAdded:Wait()
	end
	local charPosition = character:GetPivot().Position
	
	local boxSize = Vector3.new(20, 20, 20)
	local boxCFrame = CFrame.new(charPosition + Vector3.new(10, 0, 0), charPosition)
	
	local parts = game.Workspace:GetPartBoundsInBox(boxCFrame, boxSize)
	for _, part in pairs(parts) do
		if part ~= nil and part:IsA("BasePart") then -- just to make sure :>
			local partVelocity = part.AssemblyLinearVelocity
			local partPosition = part.Position
			local partToCharVector = charPosition - partPosition
			local partToCharDistance = partToCharVector.Magnitude
			local partToCharUnit = partToCharVector.Unit
			
			if partVelocity.Magnitude > 0 and partToCharDistance < 20 then
				if partVelocity:Dot(partToCharUnit) > 0 then
					character:WaitForChild("Head"):Destroy()
				end
			end
		end
	end
	
	task.wait()
end