F# Using Pose to Shim printfn for unit testing - unit-testing

Question

I'm trying to use Pose (https://github.com/tonerdo/pose) to shim out printfn so I can unit test what is passed to it. I've managed to get it working if I'm passing a TextWriterFormat that doesn't take additional arguments, but when I try to shim one that does take additional arguments I get a Mismatched instance types exception
This works:
let workingPrintfTest () =
let a = Printf.TextWriterFormat<unit>("herp")
Printf.printfn a
let workingShim =
Shim
.Replace(fun () ->
Printf.printfn(Is.A<Printf.TextWriterFormat<unit>>())
)
.With(Func<Printf.TextWriterFormat<unit>, unit>(fun s ->
Console.WriteLine("Hijacked: " + s.ToString())
))
PoseContext.Isolate ((fun () ->
Program.workingPrintfTest () // Hijacked: herp
), workingShim)
However, when I change herp to herp: %s and the TextWriterFormat becomes TextWriterFormat<string -> unit> is when I get the mismatched instance types exception.
This throws exception:
let printfTest toPrint =
let a = Printf.TextWriterFormat<string -> unit>("herp: %s")
Printf.printfn a toPrint
let shim =
Shim
.Replace(fun () ->
let a = Is.A<Printf.TextWriterFormat<string -> unit>>()
Printf.printfn a (Is.A<string>())
)
.With(Func<Printf.TextWriterFormat<string -> unit>, unit>(fun s ->
Console.WriteLine("Hijacked: " + s.ToString())
()
)
)
PoseContext.Isolate ((fun () ->
Program.printfTest "www"
), shim)
And the accompanying Exception: Pose.Exceptions.InvalidShimSignatureException: Mismatched instance types
I'm running this in .Net Core 2.2
Any help would be appreciated.