Script that saves Players table outputs error?

I have a script that saves a players table using data store but whenever I save the players table it comes out with this error.

"Cannot convert mixed or non-array tables: keys must be strings" Here are my scripts:

Local script:

for i,v in pairs(KnifesFrame.ShopFrame:GetChildren()) do
	if v:IsA("ImageButton") then
		 click = v.MouseButton1Click:connect(function()
			KnifesFrame.KnifeName.Text = v.KnifeName.Value
			KnifesFrame.Knife.Image = v.PicID.Value
			KnifesFrame.Knife.Visible = true
			KnifesFrame.Buy.Visible = true
			KnifesFrame.Buy.Active = true
			KnifesFrame.Buy.MouseButton1Click:connect(function()
				RemoteEvents.KniveEvents.BuyKnife:FireServer(v.KnifeName.Value,v.Amount.Value)
				RemoteEvents.KniveEvents.BuyKnife.OnClientEvent:connect(function(success)
					if success == true then
						table.insert(Knifes,#Knifes,v.KnifeName.Value)
						RemoteEvents.KniveEvents.SaveKnives:FireServer(Knifes) -- THis is where I am getting told that the problem is but I have no idea whats wrong?
						KnifesFrame.KnifeName.Text = "Success!"
						wait(2)
						KnifesFrame.KnifeName.Text = ""
						KnifesFrame.Buy.Visible = false
						KnifesFrame.Buy.Active = false
						click:disconnect()
					else
						KnifesFrame.Buy.Visible = false
						KnifesFrame.Buy.Active = false
						KnifesFrame.KnifeName.Text = "You dont have enough points for this item!"
						wait(2)
						KnifesFrame.KnifeName.Text = ""
						click:disconnect()
					end
				end)
			end)
		end)
	end
end