BigInt Bug or just me?

[Tyro[17]]

Why does the following implementation of the binomial coefficient 
yield two different answers?

import std.stdio, std.bigint;

void main()
{
     // Correct result when using long
     writeln("(40  20) = ", binomial(40L, 20L));

     // 2 times the expected result when using BigInt
     writeln("(40  20) = ", binomial(BigInt(40), BigInt(20))/2);
}

T binomial(T)(T n, T k)
{
     T iter(T n, T k, T i, T prev)
     {
         if (i >= k) return prev;
         return iter(n, k, i+1, ((n-i)*prev)/(i+1));
     }

     if (k > (n-1)) return iter(n, k, cast(T)0, cast(T)1);
     return iter(n, (n-k), cast(T)0, cast(T)1);
}

Additionally, why is there no implicit conversion from integer to 
BigInt?
Surely now precision will be lost when performing this 
conversion. All
those casts are butt ugly if you ask me. I believe one should be 
able to
assign any integral value to BigInt and reasonably expect that it 
be
implicitly converted.

Thanks,
Andrew