Fixing #1644

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Collections/SeqModule2.fs

@@ -918,6 +918,20 @@ type SeqModule2() =
         for i = 11 to 20 do
            CheckThrowsArgumentException (fun () -> Seq.item i { 10 .. 20 } |> ignore)
 
+    [<Test>]
+    member this.``item should fail with correct number of missing elements``() =
+        try
+            Seq.item 0 (Array.zeroCreate<int> 0) |> ignore
+            failwith "error expected"
+        with
+        | exn when exn.Message.Contains("seq was short by 1 element") -> ()
+
+        try
+            Seq.item 2 (Array.zeroCreate<int> 0) |> ignore
+            failwith "error expected"
+        with
+        | exn when exn.Message.Contains("seq was short by 3 elements") -> ()
+
     [<Test>]
     member this.Of_Array() =
          // integer Seq

src/fsharp/FSharp.Core/seq.fs

@@ -64,9 +64,10 @@ namespace Microsoft.FSharp.Collections
 
       let rec nth index (e : IEnumerator<'T>) =
           if not (e.MoveNext()) then
+            let shortBy = index + 1
             invalidArgFmt "index"
                 "{0}\nseq was short by {1} {2}"
-                [|SR.GetString SR.notEnoughElements; index; (if index=1 then "element" else "elements")|]
+                [|SR.GetString SR.notEnoughElements; shortBy; (if shortBy = 1 then "element" else "elements")|]
           if index = 0 then e.Current
           else nth (index-1) e