Debug symbols not loaded when converting to .net 4

I have a .net 3.5 project in C# that hits breakpoints fine when I F5 it in Visual Studio 2010. However, when I change the project target framework to .Net4, and hit F5, then the breakpo开发者_开发技巧ints are not hit. If I mouse over the breakpoint icon i see that no debug symbols have been loaded for this document. If I change the target framework back to .Net3.5, breakpoints are hit as expected. Any pointers as to where I should look for why this is?

Click Debug, Windows, Modules, right-click your assembly, and click Symbol Load Info.