diff --git a/src/MimeMapping/KnownMimeTypes.cs b/src/MimeMapping/KnownMimeTypes.cs
index d18b594..54008a7 100644
--- a/src/MimeMapping/KnownMimeTypes.cs
+++ b/src/MimeMapping/KnownMimeTypes.cs
@@ -7149,7 +7149,7 @@ public static class FileExtensions
 
 
         // Switch-case instead of dictionary since it does the hashing at compile time rather than run time
-        internal static string LookupType(string type)
+        internal static string? LookupType(string type)
         {
             switch (type)
             {
@@ -10213,7 +10213,7 @@ internal static string LookupType(string type)
         }
 
         // Switch-case instead of dictionary since it does the hashing at compile time rather than run time
-        internal static string[] LookupMimeType(string mimeType)
+        internal static string[]? LookupMimeType(string mimeType)
         {
             switch (mimeType)
             {
diff --git a/src/MimeMapping/KnownMimeTypes.tt b/src/MimeMapping/KnownMimeTypes.tt
index e9c8d33..6bd9fab 100644
--- a/src/MimeMapping/KnownMimeTypes.tt
+++ b/src/MimeMapping/KnownMimeTypes.tt
@@ -155,7 +155,7 @@ namespace MimeMapping
 
 
         // Switch-case instead of dictionary since it does the hashing at compile time rather than run time
-        internal static string LookupType(string type)
+        internal static string? LookupType(string type)
         {
             switch (type)
             {
@@ -181,7 +181,7 @@ namespace MimeMapping
         }
 
         // Switch-case instead of dictionary since it does the hashing at compile time rather than run time
-        internal static string[] LookupMimeType(string mimeType)
+        internal static string[]? LookupMimeType(string mimeType)
         {
             switch (mimeType)
             {
diff --git a/src/MimeMapping/MimeMapping.csproj b/src/MimeMapping/MimeMapping.csproj
index d871b31..f0b2313 100644
--- a/src/MimeMapping/MimeMapping.csproj
+++ b/src/MimeMapping/MimeMapping.csproj
@@ -3,6 +3,8 @@
   <PropertyGroup>
     <!-- https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting -->
     <TargetFrameworks>netstandard2.0;net462</TargetFrameworks>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
     <Description>Constants for (almost) all MIME types and method to determine MIME type from a file name.
 Contains just over 1000 mime types.
 
diff --git a/src/MimeMapping/MimeUtility.cs b/src/MimeMapping/MimeUtility.cs
index 2da12ba..c452cc7 100644
--- a/src/MimeMapping/MimeUtility.cs
+++ b/src/MimeMapping/MimeUtility.cs
@@ -16,23 +16,23 @@ public static class MimeUtility
         /// </summary>
         public const string UnknownMimeType = "application/octet-stream";
 
-        private static readonly Lazy<ReadOnlyDictionary<string, string>> _lazyDictExtensions = new Lazy<ReadOnlyDictionary<string, string>>(
-            () => new ReadOnlyDictionary<string, string>(KnownMimeTypes.ALL_EXTS.Value.ToDictionary(e => e, e => KnownMimeTypes.LookupType(e)))
+        private static readonly Lazy<ReadOnlyDictionary<string, string?>> _lazyDictExtensions = new Lazy<ReadOnlyDictionary<string, string?>>(
+            () => new ReadOnlyDictionary<string, string?>(KnownMimeTypes.ALL_EXTS.Value.ToDictionary(e => e, e => KnownMimeTypes.LookupType(e)))
         );
 
-        private static readonly Lazy<ReadOnlyDictionary<string, string[]>> _lazyDictMimeTypes = new Lazy<ReadOnlyDictionary<string, string[]>>(
-            () => new ReadOnlyDictionary<string, string[]>(KnownMimeTypes.ALL_MIMETYPES.Value.Distinct().ToDictionary(e => e, e => KnownMimeTypes.LookupMimeType(e)))
+        private static readonly Lazy<ReadOnlyDictionary<string, string[]?>> _lazyDictMimeTypes = new Lazy<ReadOnlyDictionary<string, string[]?>>(
+            () => new ReadOnlyDictionary<string, string[]?>(KnownMimeTypes.ALL_MIMETYPES.Value.Distinct().ToDictionary(e => e, e => KnownMimeTypes.LookupMimeType(e)))
         );
 
         /// <summary>
         /// Dictionary of all available types by extension (lazy loaded on first call)
         /// </summary>
-        public static ReadOnlyDictionary<string, string> TypeMap => _lazyDictExtensions.Value;
+        public static ReadOnlyDictionary<string, string?> TypeMap => _lazyDictExtensions.Value;
         
         /// <summary>
         /// Dictionary of all available types by mimetype (lazy loaded on first call)
         /// </summary>
-        public static ReadOnlyDictionary<string, string[]> TypeToExtensionsMap => _lazyDictMimeTypes.Value;
+        public static ReadOnlyDictionary<string, string[]?> TypeToExtensionsMap => _lazyDictMimeTypes.Value;
 
         /// <param name="file">The file extensions (ex: "zip"), the file name, or file path</param>
         /// <returns>The mime type string, returns "application/octet-stream" if no known type was found</returns>
@@ -53,7 +53,7 @@ public static string GetMimeMapping(string file)
 
         /// <param name="mimeType">The mime type string, e.g. "application/json"</param>
         /// <returns>One or more extensions matching the mime type or null if no match</returns>
-        public static string[] GetExtensions(string mimeType)
+        public static string[]? GetExtensions(string mimeType)
         {
             if (string.IsNullOrEmpty(mimeType)) throw new ArgumentNullException(mimeType);