Changing Humanoid properties after a DevProduct is purchased?

I literally just asked a question but I'm making a game and sadly I keep running into issues.

So this would increase the purchaser's WalkSpeed by 7 once purchased. Here is the total script -

local MartketplaceService = game:GetService('MarketplaceService')
local devproductid = 84718040
local devproductid2 = 84717927
local devproductid3 = 84715500
MartketplaceService.ProcessReceipt = function(receiptInfo)
	for i, player in ipairs(game.Players:GetChildren())do
		if player.userId == receiptInfo.PlayerId then
			if receiptInfo.ProductId == devproductid2 then
				player.leaderstats.Stage.Value = player.leaderstats.Stage.Value + 1
			end
			if receiptInfo.ProductId == devproductid then
				player.leaderstats.Stage.Value = player.leaderstats.Stage.Value + 3
			end
			if player.leaderstats.Stage.Value > 15 then
				player.leaderstats.Stage.Value = 15
			end
			if receiptInfo.ProductId == devproductid3 then
				player:FindFirstChild("Character").Humanoid.WalkSpeed = player:FindFirstChild("Character").Humanoid.WalkSpeed + 7
			end
		end
	end
	return Enum.ProductPurchaseDecision.PurchaseGranted
end

But this is the part that is not working and causing me problems. The error that is popping up is "Line 18 is indexing a nil value" -

			if receiptInfo.ProductId == devproductid3 then
				player:FindFirstChild("Character").Humanoid.WalkSpeed = player:FindFirstChild("Character").Humanoid.WalkSpeed + 7
			end