This was appearing on a very simple unit test and no obvious stack trace was available and i couldn't debug/step into it.
The problem was that I had added a parameter to the constructor, and Nunit no longer had the default/blank constructor to call.
This is an example of a broken constructor
public AccountsServiceTests(string s)
: base()
{
}
Removing the parameter will fix the problem like so...
public AccountsServiceTests()
: base()
{
}