TypeError: callback is not a function

[ishan9678]

Client:

socket?.emit(SESSION_EVENTS.START, {}, (ack: { sessionCode?: string; error?: string }) => {
        if (ack.error) {
          setConnectionStatus('disconnected')
          sendResponse({ error: ack.error })
        } else if (ack.sessionCode) {
          currentSessionCode = ack.sessionCode
          sessionState = 'hosting'
          setConnectionStatus('connected')
          sendResponse({ sessionCode: ack.sessionCode })
        }
      })

server:

socket.on(SESSION_EVENTS.START, (_, callback) => {
    console.log("callback", callback)
    const sessionCode = generateCode()
    sessions[sessionCode] = socket.id
    socket.join(sessionCode)
    updateRoomSize(sessionCode)
    console.log(`Host ${socket.id} started ${sessionCode}`)

    if (typeof callback === 'function') {
        callback({ success: true, sessionCode })
    }
  })

Terminal output

callback [Function (anonymous)]
Room 991583 now has 1 peers
Host Lti-_PgqnZDuEESBAAAB started 991583
TypeError: callback is not a function

Not sure what is going wrong here, i'm on socket io latest version (^ symbol).
It even logs callback as a function and passes the if condition but throws this error