F# Warning FS0049 when catching an exception class

ExceptionWarning.fs

type SitAction = { Rest: int }
type StandAction = { Rest: int }
type Action =
    | Stand of StandAction
    | Sit of SitAction

let lastStandIndex (x: Action list) : int option =
    try
        x |> List.tryFindIndex (function
            | Stand _ -> true
            | _ -> false)
    with
        | KeyNotFoundException -> None
        // Warning FS0049: Uppercase variable identifiers should not generally be used in patterns,
        // and may indicate a missing open declaration or a misspelt pattern name. 
        
        // How do I avoid this warning without suppressing it?
        // I tried using the full name System.Collections.Generic.KeyNotFoundException, but that would not compile.

You need to pattern-match with the type since this is what KeyNotFoundException is:
| :? KeyNotFoundException as ex -> None

And open System.Collections.Generic up top to import the definition

Perfect, thank you!
(I have left out the as ex part, since I'm not going to use the identifier.)