Haskell syntax for a case expression in a do block

I can't quite figure out this syntax problem with a case expression in a do block.

What is the correct syntax?

If you could correct my example and explain it that would be the best.

module Main where 

main = do   
     putStrLn "This is a test"
     s <- foo
     putStrLn s  

foo = do
    args <- getArgs 
    return case args of
                [] -> "No Args"
                [s]-> "Some Args"

A little update. My source file was a mix of spaces and tabs and it was causing all kinds of problems. Just a tip for any one else starting in Haskell. If you are having problems check for tabs and spaces in your source code.