NullReferenceException in VsDebugPresentationPackage.dll

When I develop my DebugEngine with Visual Studio 2010 shell (isolated), I meet this exception.

That happened certainly in following steps:

1.Make a breakpoint in .c source file;

2.Click RuntoCursor in DisassemblyWindow and it will stop on the location where the breakpoint is;

3.Disable this breakpoint and click RuntoCursor in DisassemblyWindow again;

4.A Exception Dialogue popped up.

When This breakpoint make in DisassemblyWindow, it works well, While in .c source file, System.NullReferenceException

will pop up... I would like to debug VsDebugPresentationPackage.dll, where can I get .PDB file..

Otherwise , how can I troubleshoot this problem? Any one can help...