[ASP.Net] Security Exception (partially trust callers) in Medium Trust

Security Exception

Description: The application attempted to perform an operation not allowed by the security policy.  To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file.

Exception Details: System.Security.SecurityException: That assembly does not allow partially trusted callers.

Imports ICSharpCode.SharpZipLib.Zip
Imports System.IO
Imports System.Drawing
Imports System.Drawing.Imaging

Partial Class CMS_AddFoto
    Inherits System.Web.UI.Page


    Protected Sub btnUpload_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnUpload.Click
	Dim s As ZipInputStream
        Dim theentry As ZipEntry
        Dim filename As String

        s = New ZipInputStream(File.OpenRead(Server.MapPath("temp.zip")))
        theentry = s.GetNextEntry

        Do Until theentry Is Nothing
            filename = Path.GetFileName(theentry.Name)
            Unzip("temp.zip", "Temp\", filename)
            theentry = s.GetNextEntry
        Loop

[...]

    End Sub

    Public Sub Unzip(ByVal ZipFile As String, ByVal UnzipToFolder As String, ByVal Extract_File As String)
        Dim z_file As New ZipFile(File.OpenRead(Server.MapPath(ZipFile)))
        Dim z_entry As ZipEntry = z_file.GetEntry(Extract_File)
        Dim nbyte As Int32 = 0
        Dim read_stream As Stream = z_file.GetInputStream(z_entry)
        Dim write_stream As New FileStream(Path.Combine(Server.MapPath(UnzipToFolder), z_entry.Name), FileMode.Create)
        Dim bin_write As New BinaryWriter(write_stream, System.Text.Encoding.ASCII)
        Dim zip_data(1024) As Byte

        Do
            nbyte = read_stream.Read(zip_data, 0, 1024)
            If nbyte = 0 Then Exit Do
            bin_write.Write(zip_data, 0, nbyte)
        Loop

        read_stream.Close()
        bin_write.Close()
        write_stream.Close()
End Sub
End Class

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I'm using version 0.84 of the #ziplib.

I would really appreciate any help on how to get things work.