Why can't I index a std::vector in the immediate window?

So, I have a vector

 std::vector<std::string> lines. 

I fill this vector up, and can access it like

 std::string temp = lines[0];

However, in the immediate window, both

 lines[0] - error:overloaded operator not found

and

 lines.at(0) - error:symbol is ambiguous

don't work at all. Is there a trick to using the immediate window with c++. I'm mostly coming from a C# background, where everything works nicely (and I have intellisense in the Immediate Window). I wasn't expecting C++ to be great, but I figured it would work for things besides ints. Can anyone tell me what I'm doing wrong? Thanks.

EDIT: I should be clear, nothing really works in the immediate window, this is just a simplified example

EDIT: I'm in debug mode