Number Expected Error

I made a program for a turtle to turn gravel into flint. It worked fine until the part where it drops all but one flint into a chest above it.

turtle.select(1)
gravel = turtle.getItemCount(1)
while true do
turtle.place()
turtle.dig()
gravel = turtle.getItemCount(1)
if tonumber(gravel) < 1 then
  turtle.select(16)
  while turtle.getItemCount() < 1 do
   turtle.dropUp(1)
  end
  turtle.select(1)
  os.reboot()
end
end

The exact error was: gravel :9: Expected number

As can be seen on the wiki turtle.getItemCount requires the slot number to get the count of

Thanks.

Also you don't need

tonumber(gravel)

as gravel is already a number simply use

if gravel < 1 then

.