Parallel processing and further use of output

[Zoidberg via Digitalmars-d-learn]

I've run into an issue, which I guess could be resolved easily, 
if I knew how...

[CODE]
     ulong i = 0;
     foreach (f; parallel(iota(1, 1000000+1)))
     {
         i += f;
     }
     thread_joinAll();
     i.writeln;
[/CODE]

It's basically an example which adds all the numbers from 1 to 
1000000 and should therefore give 500000500000. Running the above 
code gives 205579930677, leaving out "thread_joinAll()" the 
output is 210161213519.

I suspect there's some sort of data race. Any hint how to get 
this straight?