Get last part of URL after last slash

Hello,
I am trying to extract the last part of the URL after the final slash but am having problems doing so using the code below. What am I doing wrong here please?
Dim fullUrl As String fullUrl = Request.Url.ToString()
Dim topic_start As String topic_start = fullUrl.LastIndexOf("/") + 1
Dim topic As String = fullUrl.Substring(topic_start, fullUrl.Length - topic_start)