diff --git a/src/fsharp/tast.fs b/src/fsharp/tast.fs
index 8dc55d5f8b..25cfc4a7b6 100644
--- a/src/fsharp/tast.fs
+++ b/src/fsharp/tast.fs
@@ -35,12 +35,12 @@ open Microsoft.FSharp.Core.CompilerServices
 /// Unique name generator for stamps attached to lambdas and object expressions
 type Unique = int64
 //++GLOBAL MUTABLE STATE
-let newUnique = let mutable i = 0L in fun () -> System.Threading.Interlocked.Increment(&i)
+let newUnique = let i = ref 0L in fun () -> System.Threading.Interlocked.Increment(i)
 type Stamp = int64
 
 /// Unique name generator for stamps attached to to val_specs, tycon_specs etc.
 //++GLOBAL MUTABLE STATE
-let newStamp = let mutable i = 0L in fun () -> System.Threading.Interlocked.Increment(&i)
+let newStamp = let i = ref 0L in fun () -> System.Threading.Interlocked.Increment(i)
 
 /// A global generator of compiler generated names
 // ++GLOBAL MUTABLE STATE