diff --git a/src/Compiler/Driver/fsc.fs b/src/Compiler/Driver/fsc.fs
index a6cd7733b98..6a537c62d64 100644
--- a/src/Compiler/Driver/fsc.fs
+++ b/src/Compiler/Driver/fsc.fs
@@ -908,7 +908,8 @@ let main3
                     use s = ilResource.GetBytes().AsStream()
                     let sha256 = System.Security.Cryptography.SHA256.Create()
                     sha256.ComputeHash s)
-                |> List.sumBy hash
+                |> List.sumBy (hash >> int64)
+                |> hash
 
             try
                 Fsharp.Compiler.SignatureHash.calculateSignatureHashOfFiles typedImplFiles tcGlobals observer