Type mismatch with Async in F#

I’ve just started messing around with F# and am trying to do some basic parallel computation to get familiar with the language. I’m having trouble with type mismatches. Here’s an example:

let allVariances list =
    seq {
        for combination in allCombinations list do
            yield (combination, abs(targetSum - List.sum combination))

let compareVariance tup1 tup2 =
    if snd tup1 < snd tup2 then

let aCompareVariance tup1 tup2 =
    async { return compareVariance tup1 tup2 }

let matchSum elements targetSum =
    allVariances elements
    |> Seq.reduce aCompareVariance
    |> Async.Parallel
    |> Async.RunSynchronously

So, “allVariances elements” produces a seq<float list * float>. CompareVariance takes two of those <float list * float> tuples and returns the one with the smaller second item (variance). My goal is to use Reduce to end up with the tuple with the smallest variance. However, I get a type mismatch on the aCompareVariance argument:

Error 1 Type mismatch. Expecting a float list * float -> float list * float -> float list * float but given a float list * float -> float list * float -> Async<float list * float> The type ‘float list * float’ does not match the type ‘Async<float list * float>’

It seems like the Async return type isn’t accepted by Reduce?

Type mismatch with Async in F#