function MidB not recognized ?!

[Njoannides]

I wrote this in ASP :

<%@ Language="VBScript">
<%
Dim TotalBytes, DataRead
Dim PosBeg, PosEnd, Eof
	   
TotalBytes = Request.TotalBytes
DataRead = Request.BinaryRead(n)
PosBeg = 1
PosEnd = InstrB( PosBeg, DataRead, Chr(13) )
Eof = MidB( DataRead, PosBeg, (PosEnd-PosBeg) )
%>

And Internet Explorer 4.0 answers me:

Erreur d'exécution Microsoft VBScript erreur '800a0005'
Appel ou argument de procédure incorrect: 'MidB'

WHY ?

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