execute command setblock

Ok so when you throw a snowball I have

/execute @e[type=Snowball] /setblock ~ ~-1 ~ minecraft:ice' 

So this makes an ice path following under the snowball. I had some code something like this and it worked but I modified it and accidently broke it. So what is the correct code that I originally had that made this work? I also tried everything like /execute @e[type=Snowball] /setblock minecraft:ice ~ ~-1 ~ and

/execute ~ ~-1 ~ @e[type=Snowball] /setblock minecraft:ice last one I tried was /execute @e[type=Snowball] ~ ~-1 ~ /setblock minecraft:ice

/execute @e[type=Snowball] ~ ~ ~ /setblock ~ ~-1 ~ ice

I tested that and it works perfectly fine. I think you do need that / in front of setblock for it to work.