diff --git a/src/libraries/System.Net.Http/tests/UnitTests/Headers/ContentDispositionHeaderValueTest.cs b/src/libraries/System.Net.Http/tests/UnitTests/Headers/ContentDispositionHeaderValueTest.cs
index cb44f49b767814..2d411c36e2cf71 100644
--- a/src/libraries/System.Net.Http/tests/UnitTests/Headers/ContentDispositionHeaderValueTest.cs
+++ b/src/libraries/System.Net.Http/tests/UnitTests/Headers/ContentDispositionHeaderValueTest.cs
@@ -1,14 +1,39 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Buffers;
 using System.Collections.Generic;
 using System.Linq;
 using System.Net.Http.Headers;
-
 using Xunit;
+using Xunit.Abstractions;
 
 namespace System.Net.Http.Tests
 {
+    public class Encode5987ReproTest
+    {
+        private static readonly IndexOfAnyValues<byte> s_rfc5987AttrBytes =
+            IndexOfAnyValues.Create("!#$&+-.0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ^_`abcdefghijklmnopqrstuvwxyz|~"u8);
+
+        private readonly ITestOutputHelper _output;
+
+        public Encode5987ReproTest(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        [Fact]
+        public void IndexOfAnyExceptTest()
+        {
+            ReadOnlySpan<byte> bytes = new byte[] { 70, 105, 108, 101, 195, 131, 78, 97, 109, 101, 46, 98, 97, 116 };
+
+            int offset = bytes.IndexOfAnyExcept(s_rfc5987AttrBytes);
+            _output.WriteLine($"s_rfc5987AttrBytes is {s_rfc5987AttrBytes.GetType().FullName}");
+
+            Assert.Equal(4, offset);
+        }
+    }
+
     public class ContentDispositionHeaderValueTest
     {
         [Fact]
diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
index 58a2a92ef19195..e9e80916061a61 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
@@ -17,6 +17,7 @@ namespace System
 {
     public partial class String
     {
+        // Hi
         private const int StackallocIntBufferSizeLimit = 128;
 
         private static void FillStringChecked(string dest, int destPos, string src)