How to create a generator with a fixed list of items for FsCheck

I originally tried to create a generator that have the first 5 elements fixed (and on any test using Prop.forAll the first five would always run), but failed in doing so.

Now I am trying to simplify this by having one generator for random data within a range, and one generator for non-random data, i.e., a fixed sequence. It is similar to Gen.constant, except that instead of one value, it is a sequence of values.

I have this (simplified reproducible example, works with NUnit and xUnit):

[<Property(Verbose = true, MaxTest=5)>]
static member MultiplyIdentityCornerCases () =
    Gen.elements(Seq.ofList ([0L; -1L; 1L; Int64.MinValue; Int64.MaxValue]))
    |> Arb.fromGen 
    |> Prop.forAll <| fun x -> x = x * 1L

The output is (no idea where the null comes from):

0:
<null>
9223372036854775807L
1:
<null>
-9223372036854775807L
2:
<null>
-9223372036854775807L
3:
<null>
1L
4:
<null>
-9223372036854775807L
Ok, passed 5 tests.

I'd like the output to contain all the five tests in the sequence, preferably, but not necessarily, in order. I know I can do this with NUnit (or any unit testing system) using a testdata provider, but I wonder whether I can do it with FsCheck (or whether I should, perhaps this is a bad idea).

I think using FsCheck is useful, as for the situation where there's more than one function argument, I want it to exhaustively test all combinations of the corner cases arguments I give it. This is hopefully easier with FsCheck than with a testdata provider.