Turtle, redstone placement?

i've tryed to let a turtle mine a block and place redstone instead of it but what he does is, kill the block under him and replace it
this is my code:

while turtle.detectDown(grass) do
turtle.digDown(grass)
turtle.placeDown(redstone)
turtle.forward()
end

if anyone could help, i know a little bit about this since i started yesterday but i already accomplished making a autofarmer by watching other codes (not copy pasting) and learning out of it

turtle.detectDown and turtle.placeDown dont take any parameters. When you use placeDown you need to use turtle.select(slot number) to select the slot of the turtle's inventory with the redstone