Writeline problem

I have this small program, when I run it and it shows an error as " Conversion from string "firstname: Khoi.firstname" to type 'Integer' is not valid"

Please advise. Thanks
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Module Module1

Structure Profile
Dim firstname As String
Dim lastname As String
Dim address As String
Dim dob As Date
End Structure

Sub Main()
Dim Khoi As New Profile()
Khoi.firstname = "Quang Khoi"
Khoi.lastname = "Nguyen Truong"
Khoi.address = "196/16A De Tham"
Khoi.dob = "Sep 20,1982"
WriteLine("firstname:" + " " + "Khoi.firstname")
WriteLine("lastname:" + " " + "Khoi.lastname")
WriteLine("address:" + " " + "Khoi.address")
WriteLine("dob:" + " " + "Khoi.dob")

End Sub

End Module
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