Haskell IO: Couldn't match expected type `IO a0' with actual type

I am new to Haskell, and I try to understand how to do IO correctly.

The following works ok:

main = do
  action <- cmdParser
  putStrLn "Username to add to the password manager:"
  username <- getLine
  case action of
    Add -> persist entry
      where
        entry = Entry username "somepassword"

Whereas the following results in compilation error:

main = do
  action <- cmdParser
  case action of
    Add -> persist entry
      where
        entry = Entry promptUsername "somepassword"

promptUsername = do
  putStrLn "Username to add to the password manager:"
  username <- getLine

The error is here:

Couldn't match expected type `IO b0' with actual type `[Char]'
Expected type: IO b0
  Actual type: String
In the expression: username
[...]

What is going on here? Why the first version works, while the second one does not?

I know that in Stack Overflow there are a few similar questions like this, but none of them seemed to explain this problem to me.