IMAP's `fetch` results with extreme memory usage and computer freezing.

I have written the following function using the HaskellNet and HaskellNet-SSL libraries.
Using the "fetch" function results with memory usage blowing up and extreme swapping.
What am I doing wrong? Might it be a bug?

import qualified Data.ByteString.Char8       as SB8
import           Network.HaskellNet.IMAP
import           Network.HaskellNet.IMAP.SSL

main = retrieveCSV

retrieveCSV = do
    con <- connectIMAPSSL imapServer
    login con username password
    select con mailbox
    lastMsg <- last <$> search con [ALLs]
    -- this works:
    _ <- fetchHeader con lastMsg
    -- this works:
    _ <- fetchSize con lastMsg
    -- this blows up in memory usage:
    _ <- fetch con lastMsg
    undefined
    where
    imapServer = "imap.gmail.com"
    username   = "example" -- imagine this username is real
    password   = "example" -- imagine this password is real
    mailbox    = "example" -- imagine this mailbox is real

Start by reproducing retrieveCSV with fetch con lastMsg and compare its memory use with fetchHeader and fetchSize. Trace the HaskellNet IMAP fetch entry point to determine whether the library or caller causes the excessive allocation; done means establishing the cause and verifying a focused fix or documented explanation.