diff --git a/src/fsharp/FSharp.Core/string.fs b/src/fsharp/FSharp.Core/string.fs
index d009615e6aa..5c8f15ba717 100644
--- a/src/fsharp/FSharp.Core/string.fs
+++ b/src/fsharp/FSharp.Core/string.fs
@@ -52,13 +52,19 @@ namespace Microsoft.FSharp.Core
 
         [<CompiledName("MapIndexed")>]
         let mapi (mapping: int -> char -> char) (str:string) =
-            if String.IsNullOrEmpty str then
+            let len = length str
+            if len = 0 then 
                 String.Empty
             else
-                let res = StringBuilder str.Length
+                let result = str.ToCharArray()
                 let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(mapping)
-                str |> iteri (fun i c -> res.Append(f.Invoke(i, c)) |> ignore)
-                res.ToString()
+
+                let mutable i = 0
+                while i < len do
+                    result.[i] <- f.Invoke(i, result.[i])
+                    i <- i + 1
+
+                new String(result)
 
         [<CompiledName("Filter")>]
         let filter (predicate: char -> bool) (str:string) =
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs
index 6c1b1360585..5470b6dece6 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs
@@ -103,11 +103,35 @@ type StringModule() =
 
     [<Test>]
     member this.MapI() =
-        let e1 = String.mapi (fun i c -> char(int c + i)) "foo"
-        Assert.AreEqual("fpq", e1)
+        let e1 = String.mapi (fun _ c -> c) "12345"
+        Assert.AreEqual("12345", e1)
 
-        let e2 = String.mapi (fun i c -> c) null 
-        Assert.AreEqual("", e2)
+        let e2 = String.mapi (fun _ c -> c + char 1) "1"
+        Assert.AreEqual("2", e2)
+
+        let e3 = String.mapi (fun _ c -> c + char 1) "AB"
+        Assert.AreEqual("BC", e3)
+
+        let e4 = String.mapi (fun i c -> char(int c + i)) "hello"
+        Assert.AreEqual("hfnos", e4)
+
+        let e5 = String.mapi (fun _ c -> c) null 
+        Assert.AreEqual("", e5)
+
+        let e6 = String.mapi (fun _ c -> c) String.Empty 
+        Assert.AreEqual("", e6)
+
+        let e7 = String.mapi (fun _ _ -> failwith "should not fail") null 
+        Assert.AreEqual("", e7)
+
+        let e8 = String.mapi (fun i _ -> if i = 1 then failwith "should not fail" else char i) "X" 
+        Assert.AreEqual("\u0000", e8)
+
+        // side-effect and "order of operation" test
+        let mutable x = 0
+        let e9 = String.mapi (fun i c -> x <- x + i; c + char x) "abcde"
+        Assert.AreEqual(x, 10)
+        Assert.AreEqual(e9, "acfjo")
 
     [<Test>]
     member this.Filter() =