String.Substring(start, end) sometimes throwing an exception

My program uses a BackgroundWorker to call a PerformAction() method when a different method, Method1 returns true. I also am using the Strategy Pattern to set the correct PerformAction() that should be performed every time the event is raised.

For the Strategy Pattern I created an Abstract Class, and then a class inside the abstract class that inherits it.

Public  MustInherit Class Abstract
    Public MustOverride Sub PerformAction(ByVal str as String)
    Public Class Extends
           Inherits Abstract
            Public Overrides Sub PerformAction(ByVal str as String)
                s.Substring(s.IndexOf(":"), s.IndexOf(">"))
            End Sub
End Class

I create another class that contains a field of Abstract, and that is used to call PerformAction.

The PerformAction method gets called from the BackgroundWorker.ReportProgress event, which is called when BackgroundWorker.DoWork detects that Method1 is returning true. And with the code above, it causes a System.Reflection.TargetInvocationException with addition information Exception has been thrown by the target of an invocation.

The debugger tells me:

    this    Cannot obtain value of local or argument '<this>' as it is not available at this instruction pointer, possibly because it has been optimized away.  System.Delegate

    args    Cannot obtain value of local or argument 'args' as it is not available at this instruction pointer, possibly because it has been optimized away.    object[]

Strangely enough, when I perform (what seems to me) an identical operation with two substrings:

s = s.Substring(s.IndexOf(":"))
s = s.Substring(0, s.IndexOf(">"))

it functions perfectly.

What is the difference between these two methods? Is my inheritance set up incorrectly and that is what is causing these errors? What's going on here?

Let me know if I need to add more code to explain the situation. Thanks.