mixin assembler does not work?

[Foo via Digitalmars-d-learn]

Hey guys. Can someone explain me, why this code does only works
with the inline assembler version but not with the mixin?
Thanks in advance!

Code:
import std.stdio : writeln;
import std.conv : to;

template Vala(uint count, alias arr) {
        immutable string c = to!string(count);

        enum Vala = "asm { sub ESP, " ~ c ~ "; mov " ~ arr.stringof ~ ",
" ~ c ~ "; mov " ~ arr.stringof ~ " + 4, ESP; }";
}

void main() {
        ubyte[] a;
        writeln(a.length);
        
        static if (false) {
                asm {
                        sub ESP, 1000;  // Reserve 1000 bytes
                        mov a, 1000;    // Set a.length = 1000
                        mov a + 4, ESP; // Set &a[0] to reserved bytes
                }
        } else {
                mixin Vala!(1000, a);
        }
        
        writeln(a.length);
}