Win32 api call Error GetShortPathName in f# - winapi

Question

I can get this code to run in the interactive environment but it crashes when I run the code from debugger or from the .exe file
Forgot the error:
FatalExecutionEngineError was detected!
The runtime has encountered a fatal error. The address of the error was at 0x6c9781b0, on thread 0x1104. The error code is 0xc0000005. This error may be a bug in the CLR or in the unsafe or non-verifiable portions of user code. Common sources of this bug include user marshaling errors for COM-interop or PInvoke, which may corrupt the stack.
Using .net 4.5
open System.IO
open System.Runtime.InteropServices
open System.Text
[<DllImport("kernel32.dll",CharSet = CharSet.Auto, SetLastError=true)>]
extern uint32 GetShortPathName(
[<MarshalAs(UnmanagedType.LPWStr)>] string longpath,
[<MarshalAs(UnmanagedType.LPWStr)>] StringBuilder shortpath,
[<MarshalAs(UnmanagedType.U4)>] uint32 item)
let MakeShortName(longPath : string) =
let sb = StringBuilder()
let currPath = longPath
let item = 1024u
// let blah = ""
//win32 assigns shortPath
let blah32 = GetShortPathName(currPath, sb, item)
sb.ToString()
[<EntryPoint>]
let main argv =
let path = #"C:\dev\shortName\shortName"
let shorty = MakeShortName path
printfn "%s" shorty
let x = System.Console.ReadKey()
0
Interactive env
$ (me alt+entering the above two functions)
val GetShortPathName : string * StringBuilder * uint32 -> uint32
val MakeShortName : string -> string
$ MakeShortName #"C:\dev\shortName\shortName";;
val it : string = "C:\dev\SHORTN~1\SHORTN~1"

Answer 1

Fixed it.
I just suck # win 32
Removed marshals
See solution:
open System.IO
open System.Runtime.InteropServices
open System.Text
[<DllImport("kernel32.dll",CharSet = CharSet.Auto, SetLastError=true)>]
extern int GetShortPathName(
string longpath,
StringBuilder shortpath,
int item)
let MakeShortName(longPath : string) =
let sb = StringBuilder()
let currPath = longPath
let item = 1024
// let blah = ""
//win32 assigns shortPath
let blah32 = GetShortPathName(currPath, sb, item)
sb.ToString()
[<EntryPoint>]
let main argv =
let path = #"C:\dev\shortName\shortName"
let shorty = MakeShortName path
printfn "%s" shorty
let x = System.Console.ReadKey()
0

Answer 2

Win32 functions may expect a pre-allocated buffer of the maximum size. This code fixed the crash, for me:
let MakeShortName(longPath : string) =
let maxSize = uint32 <| longPath.Length + 16
let sb = new StringBuilder("", int32 maxSize)
let len = GetShortPathName(longPath, sb, maxSize)
sb.ToString(0, int32 len)