Wrap a call?

[JN via Digitalmars-d-learn]

    import std;

    void before()
    {
        writeln("Before");
    }
    void after()
    {
        writeln("After");
    }

    void add(int x, int y)
    {
        writeln(x + y);
    }

    template WrapCall(string call)
    {
        enum WrapCall = "before(); " ~ call ~ "; after();";
    }

    void main()
    {
        mixin WrapCall!"add(1, 2)";
    }

This doesn't work, if I change it to mixin(WrapCall!"add(1, 2")), 
it works, why?