std.algorithm.reduce on an array of structs

[Colin via Digitalmars-d-learn]

I have this test code:

struct Thing {
    uint x;
}
void main(){
    uint[] ar1 = [1, 2, 3, 4, 5];
    auto min1 = ar1.reduce!((a,b) => a < b);
    writefln("%s", min1);  // prints 1 as expected

    Thing[] ar2 = [Thing(1), Thing(2), Thing(4)];
    auto min2 = ar2.reduce!((a,b) => a.x < b.x);  //  <- Wont 
Compile
    writefln("%s", min2);
}

The line with "Wont Compile" on it has this error message:
/usr/include/dmd/phobos/std/algorithm.d(770): Error: cannot 
implicitly convert expression (__lambda2(result, 
front(_param_1))) of type bool to Thing
/usr/include/dmd/phobos/std/algorithm.d(791): Error: template 
instance t.main.reduce!((a, b) => a.x < b.x).reduce!(Thing, 
Thing[]) error instantiating
t.d(16):        instantiated from here: reduce!(Thing[])


Any idea what I'm doing wrong here?
To me, the operation on ar2 should be pretty much identical to 
ar1, except for the use of the struct.