无法在从C＃调用的C ++ / CLI使用的C DLL中设置断点

I have a native C DLL being invoked by a C++/CLI object which is the ViewModel for a WPF progam in C#. I want to set breakpoints in the C DLL. When I try to set them they are grayed out and the help balloon says I have a native C DLL being invoked by a C++/CL