Strange exception, with EXTREMELY SIMPLE toString() in a struct

[Enjoys Math via Digitalmars-d-learn]

Exception Message:
First-chance exception: std.format.FormatException Unterminated 
format specifier: "%" at 
C:\D\dmd2\windows\bin\..\..\src\phobos\std\format.d(828)

[CODE]
module set;
import std.conv;

struct Set(T) {
        string toString() const {
                auto str = "{";
                return str;
        }
}

unittest {
        import std.stdio;
        auto A = Set!int();
        writeln(A);
}
[/CODE]