Problem with a Sort Code

I have a sort code that sorts the case number field and puts it into numeric order since the field in a text field. The code is as follows:

Public Function CaseNumberSort(vntCaseNum) As String

On Error GoTo ErrorHandler 

Dim strCaseNum As String 

Dim lngSortNum As Long 

Dim strZeroPad As String 

Dim intIdx As Integer 

Dim strSortCode As String 

strSortCode = "" 

strZeroPad = String(15, "0")    ' Used to pad with leading zeros 

If IsNull(vntCaseNum) Then 

    ' Case Number is null. 

    GoTo ExitHandler 

End If 

strCaseNum = Trim(CStr(vntCaseNum)) 

If strCaseNum = "" Then 

    ' Case Number is blanks or empty string. 

    GoTo ExitHandler 

End If 

If IsNumeric(Left(strCaseNum, 1)) Then 

    ' Case Number begins with numeric digits. 

    lngSortNum = Val(strCaseNum) 

    ' Pad with leading 0's so that it sorts properly. 

    strSortCode = Right(strZeroPad & lngSortNum, Len(strZeroPad)) 

    ' Append the original Case Number to the sort code. 

    strSortCode = strSortCode & strCaseNum 

    ' We're done. 

    GoTo ExitHandler 

End If 

Next intIdx

ExitHandler:

CaseNumberSort = strSortCode 

Exit Function 

ErrorHandler:

GoTo ExitHandler 

End Function

I have the code in a query to sort the case number field. When I run the query, it does not put the numbers in proper order after 999. It placed 23-1000 after 23-100.