Getting rid of "There is no source code available for the current location."

OK, this is my own fault, but I can't seem to rescue myself.

Whenever I try to step into a class that has fields with assignments calling into .NET code, I get a dialog box that contains the text "There is no source code available for the current location.":

For instance, stepping into the constructor of the following class would give me the above message:

public class Test
{
    private Stack<String> _Dummy = new Stack<String>();
    public Test() { }
}

I assume this is because at some point I fiddled with the settings for the symbol server, but no matter what I fiddle with now, I can't seem to get rid of that message.

Where has my stupidity forgotten what it did?

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Summary of my current options:

• [ ] Enable address-level debugging
• [x] Enable Just My Code (Managed Only)
• [ ] Enable .NET framework source stepping
• [ ] Enable source server support
• Symbol file .pdb locations - empty
• Cache symbols from symbol servers to this directory: C:\temp (empty)

See if some source files are accidentally excluded. Do properties of the solution and look at the Debug Source Files section under Common Properties.