diff --git a/identity-server/src/IdentityServer/Extensions/StringsExtensions.cs b/identity-server/src/IdentityServer/Extensions/StringsExtensions.cs
index a92d2ff1b..3ecc90a9c 100644
--- a/identity-server/src/IdentityServer/Extensions/StringsExtensions.cs
+++ b/identity-server/src/IdentityServer/Extensions/StringsExtensions.cs
@@ -2,6 +2,7 @@
 // See LICENSE in the project root for license information.
 
 
+#nullable enable
 using Microsoft.AspNetCore.WebUtilities;
 using System;
 using System.Collections.Generic;
@@ -16,7 +17,7 @@ namespace Duende.IdentityServer.Extensions;
 internal static class StringExtensions
 {
     [DebuggerStepThrough]
-    public static string ToSpaceSeparatedString(this IEnumerable<string> list)
+    public static string ToSpaceSeparatedString(this IEnumerable<string>? list)
     {
         if (list == null)
         {
@@ -33,7 +34,7 @@ public static IEnumerable<string> FromSpaceSeparatedString(this string input)
         return input.Split(' ', StringSplitOptions.RemoveEmptyEntries).ToList();
     }
 
-    public static List<string> ParseScopesString(this string scopes)
+    public static List<string>? ParseScopesString(this string scopes)
     {
         if (scopes.IsMissing())
         {
@@ -53,13 +54,13 @@ public static List<string> ParseScopesString(this string scopes)
     }
 
     [DebuggerStepThrough]
-    public static bool IsMissing([NotNullWhen(false)]this string value)
+    public static bool IsMissing([NotNullWhen(false)] this string? value)
     {
         return string.IsNullOrWhiteSpace(value);
     }
 
     [DebuggerStepThrough]
-    public static bool IsMissingOrTooLong(this string value, int maxLength)
+    public static bool IsMissingOrTooLong(this string? value, int maxLength)
     {
         if (string.IsNullOrWhiteSpace(value))
         {
@@ -74,13 +75,14 @@ public static bool IsMissingOrTooLong(this string value, int maxLength)
     }
 
     [DebuggerStepThrough]
-    public static bool IsPresent([NotNullWhen(true)] this string value)
+    public static bool IsPresent([NotNullWhen(true)] this string? value)
     {
         return !string.IsNullOrWhiteSpace(value);
     }
 
     [DebuggerStepThrough]
-    public static string EnsureLeadingSlash(this string url)
+    [return: NotNullIfNotNull("url")]
+    public static string? EnsureLeadingSlash(this string? url)
     {
         if (url != null && !url.StartsWith('/'))
         {
@@ -91,7 +93,8 @@ public static string EnsureLeadingSlash(this string url)
     }
 
     [DebuggerStepThrough]
-    public static string EnsureTrailingSlash(this string url)
+    [return: NotNullIfNotNull("url")]
+    public static string? EnsureTrailingSlash(this string? url)
     {
         if (url != null && !url.EndsWith('/'))
         {
@@ -102,7 +105,8 @@ public static string EnsureTrailingSlash(this string url)
     }
 
     [DebuggerStepThrough]
-    public static string RemoveLeadingSlash(this string url)
+    [return: NotNullIfNotNull("url")]
+    public static string? RemoveLeadingSlash(this string? url)
     {
         if (url != null && url.StartsWith('/'))
         {
@@ -113,7 +117,8 @@ public static string RemoveLeadingSlash(this string url)
     }
 
     [DebuggerStepThrough]
-    public static string RemoveTrailingSlash(this string url)
+    [return: NotNullIfNotNull("url")]
+    public static string? RemoveTrailingSlash(this string? url)
     {
         if (url != null && url.EndsWith('/'))
         {
@@ -124,7 +129,7 @@ public static string RemoveTrailingSlash(this string url)
     }
 
     [DebuggerStepThrough]
-    public static string CleanUrlPath(this string url)
+    public static string CleanUrlPath(this string? url)
     {
         if (String.IsNullOrWhiteSpace(url)) url = "/";
 
@@ -137,7 +142,7 @@ public static string CleanUrlPath(this string url)
     }
 
     [DebuggerStepThrough]
-    public static bool IsLocalUrl(this string url)
+    public static bool IsLocalUrl([NotNullWhen(false)] this string? url)
     {
         // This implementation is a copy of a https://github.com/dotnet/aspnetcore/blob/3f1acb59718cadf111a0a796681e3d3509bb3381/src/Mvc/Mvc.Core/src/Routing/UrlHelperBase.cs#L315
         // We originally copied that code to avoid a dependency, but we could potentially remove this entirely by switching to the Microsoft.NET.Sdk.Web sdk.
@@ -248,7 +253,7 @@ public static string AddHashFragment(this string url, string query)
     }
 
     [DebuggerStepThrough]
-    public static NameValueCollection ReadQueryStringAsNameValueCollection(this string url)
+    public static NameValueCollection ReadQueryStringAsNameValueCollection(this string? url)
     {
         if (url != null)
         {
@@ -267,7 +272,7 @@ public static NameValueCollection ReadQueryStringAsNameValueCollection(this stri
         return new NameValueCollection();           
     }
 
-    public static string GetOrigin(this string url)
+    public static string? GetOrigin(this string? url)
     {
         if (url != null)
         {