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