Ajax AutoComplete Extender with ContextKeye not working

Ajax AutoComplete Extender with ContextKeye not working

ASPX CODE:
`
function setContextKey() {
$find('AutoCompExt2').set_contextKey($get("<%=TxtSyllabus.ClientID
%>").value)
alert(("<%=TxtSyllabus.ClientID %>").value)}
</script>
<asp:TextBox ID = "TxtSem" runat = "server" Width = "200px"
onkeyup="setContextKey()"></asp:TextBox>
<asp:AutoCompleteExtender ID = "AutoCompExt2" runat = "server"
MinimumPrefixLength="2" CompletionInterval="100" FirstRowSelected =
"false" TargetControlID= "TxtSem" EnableCaching = "false"
CompletionSetCount = "10" ServiceMethod = "SearchSem" UseContextKey=
"true" ></asp:AutoCompleteExtender>
VB CODE:
<System.Web.Script.Services.ScriptMethod(),
System.Web.Services.WebMethod()> _
Public Shared Function SearchSem(ByVal prefixText As String, ByVal count
As Integer, ByVal contextKey As String) As List(Of String)
Try
Dim cnn As New SqlConnection
Dim cmd As New SqlCommand`
Dim ds As New Data.DataSet
Dim SyllabusName = Mid(contextKey, 1, Len(contextKey) - 4)
Dim Year = Mid(contextKey, Len(contextKey) - 4, Len(contextKey))
cnn.ConnectionString =
ConfigurationManager.ConnectionStrings("excelconn").ToString()
cmd.CommandText = "Select Semester From MastLookup where
SyllabusName='" & SyllabusName & "' And SyllabusYear='" & Year &
"' And Semester=@SearchText + '%'"
cmd.Parameters.AddWithValue("SearchText", prefixText)
cmd.CommandType = Data.CommandType.Text
cmd.Connection = cnn
cnn.Open()
Dim Syllabus As List(Of String) = New List(Of String)
Dim sdr As SqlDataReader = cmd.ExecuteReader
While sdr.Read
Syllabus.Add(sdr("Semester").ToString)
End While
cnn.Close()
Return Syllabus
cnn.Close()
Catch ex As Exception
End Try
End Function
ERROR: I am getting contexKey value equal to null.
even my Javascript is not called when i run the project.