Unexpected "string" keyword after "@" character. Once inside code, you do not need to prefix constructs like "string" with "@"

After upgrading an ASP.NET MVC 3 project to MVC 4 I noticed a change in the Razor parser that threw a Parser Error saying: 'Unexpected "string" keyword after "@" character. Once inside code, you do not need to prefix constructs like "string" with "@"'

Firstly this has always working when it was MVC 3 and Razor v1. I may have been getting the syntax wrong all along but if the syntax allowed it was it really wrong?

What I was doing was trying to put some server code in a Razor helper with no surrounding HTML tags, like this example:

@helper Currency1000s(int? value)
{
    if(value == null)
    {
        -
    }
    else
    {
        @string.Format("{0:C0}k", value / 1000.0)
    }
}

Interestingly if I were to replace line 9 with @value all would be fine. Anyway, it is an easy enough fix, you just need to wrap the string.Format in HTML tags or the text tags as I did here:

@helper Currency1000s(int? value)
{
    if(value == null)
    {
        -
    }
    else
    {
        @string.Format("{0:C0}k", value / 1000.0)
    }
}