C++11 : left operand of comma operator has no effect

I'm getting this warning after I started using C++11 on my project

This is the piece of code that gives the warning :

    std::string errortext = "cannot find suitable conversion for %d", index;
    LogToFile(NULL, errortext);
    Assert(false && "cannot find suitable conversion");
    return NULL;

If I remove the NULL, will the piece of code still do the same thing ? Or is there any way to fix it ?