diff --git a/Benchmarks/Benchmark.cs b/Benchmarks/Benchmark.cs
index c0ac8b1..7f42590 100644
--- a/Benchmarks/Benchmark.cs
+++ b/Benchmarks/Benchmark.cs
@@ -28,7 +28,7 @@ public class StringExtensionBenchmark
         /// Benchmark for the RemoveCharacters method.
         /// </summary>
         [Benchmark]
-        public string RemoveCharacters()
+        public string? RemoveCharacters()
         {
             return input.RemoveCharacters(charactersToRemove);
         }
@@ -64,7 +64,7 @@ public int CountSubstring()
         /// Benchmark for the ReverseWords method.
         /// </summary>
         [Benchmark]
-        public string ReverseWords()
+        public string? ReverseWords()
         {
             return input.ReverseWords();
         }
@@ -91,7 +91,7 @@ public int CountLetters()
         /// Benchmark for the RemoveDuplicateCharacters method.
         /// </summary>
         [Benchmark]
-        public string RemoveDuplicateCharacters()
+        public string? RemoveDuplicateCharacters()
         {
             return input.RemoveDuplicateCharacters();
         }
diff --git a/StringExtension/Casing/Casing.cs b/StringExtension/Casing/Casing.cs
index c0b12ee..542e23f 100644
--- a/StringExtension/Casing/Casing.cs
+++ b/StringExtension/Casing/Casing.cs
@@ -25,7 +25,8 @@ public static class Casing
     /// </summary>
     /// <param name="input">The input string.</param>
     /// <returns>The input string converted to camel case.</returns>
-    public static string ToCamelCase(this string input)
+    /// <remarks>Returns <see cref="string.Empty"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static string ToCamelCase(this string? input)
     {
         return ToCamelCase(input.AsSpan());
     }
@@ -96,7 +97,8 @@ public static string ToCamelCase(this ReadOnlySpan<char> input)
     /// </summary>
     /// <param name="input">The input string.</param>
     /// <returns>The input string converted to Pascal case.</returns>
-    public static string ToPascalCase(this string input)
+    /// <remarks>Returns <see cref="string.Empty"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static string ToPascalCase(this string? input)
     {
         return ToPascalCase(input.AsSpan());
     }
@@ -156,7 +158,8 @@ public static string ToPascalCase(this ReadOnlySpan<char> input)
     /// </summary>
     /// <param name="input">The input string.</param>
     /// <returns>The input string converted to snake case.</returns>
-    public static string ToSnakeCase(this string input)
+    /// <remarks>Returns <see cref="string.Empty"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static string ToSnakeCase(this string? input)
     {
         return ToSnakeCase(input.AsSpan());
     }
@@ -183,7 +186,8 @@ public static string ToSnakeCase(this ReadOnlySpan<char> input)
     /// </summary>
     /// <param name="input">The input string.</param>
     /// <returns>The input string converted to kebab case.</returns>
-    public static string ToKebabCase(this string input)
+    /// <remarks>Returns <see cref="string.Empty"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static string ToKebabCase(this string? input)
     {
         return ToKebabCase(input.AsSpan());
     }
@@ -215,7 +219,8 @@ public static string ToKebabCase(this ReadOnlySpan<char> input)
     /// If <see langword="false"/> (the default), every word is capitalized.
     /// </param>
     /// <returns>The input string converted to title case.</returns>
-    public static string ToTitleCase(this string input, bool useEnglishMinorWordRules = false)
+    /// <remarks>Returns <see cref="string.Empty"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static string ToTitleCase(this string? input, bool useEnglishMinorWordRules = false)
     {
         return ToTitleCase(input.AsSpan(), useEnglishMinorWordRules);
     }
@@ -405,7 +410,7 @@ private static bool IsEnglishMinorWord(ReadOnlySpan<char> word)
     /// stripped or transliterated. Returns <see cref="string.Empty"/> if
     /// <paramref name="input"/> is <see langword="null"/> or empty.
     /// </remarks>
-    public static string Slugify(this string input, char separator = '-')
+    public static string Slugify(this string? input, char separator = '-')
     {
         if (string.IsNullOrEmpty(input))
         {
diff --git a/StringExtension/Linguistics/Linguistics.cs b/StringExtension/Linguistics/Linguistics.cs
index c1ec2c1..a747ab9 100644
--- a/StringExtension/Linguistics/Linguistics.cs
+++ b/StringExtension/Linguistics/Linguistics.cs
@@ -14,7 +14,8 @@ public static class Linguistics
     /// </summary>
     /// <param name="input">The input string.</param>
     /// <returns><c>true</c> if the string is a palindrome; otherwise, <c>false</c>.</returns>
-    public static bool IsPalindrome(this string input)
+    /// <remarks>Returns <see langword="false"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static bool IsPalindrome(this string? input)
     {
         return IsPalindrome(input.AsSpan());
     }
@@ -83,7 +84,8 @@ public static bool IsPalindrome(this ReadOnlySpan<char> input)
     /// </summary>
     /// <param name="input">The input string.</param>
     /// <returns>The number of letters in the input string.</returns>
-    public static int CountLetters(this string input)
+    /// <remarks>Returns <c>0</c> if <paramref name="input"/> is <see langword="null"/>.</remarks>
+    public static int CountLetters(this string? input)
     {
         return CountLetters(input.AsSpan());
     }
diff --git a/StringExtension/StringExtension.cs b/StringExtension/StringExtension.cs
index 4421662..880ceee 100644
--- a/StringExtension/StringExtension.cs
+++ b/StringExtension/StringExtension.cs
@@ -15,7 +15,7 @@ public static class StringExtension
     /// <param name="charactersToRemove">An array of characters to remove.</param>
     /// <returns>A new string with specified characters removed.</returns>
     /// <remarks>Returns <see langword="null"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
-    public static string RemoveCharacters(this string input, char[] charactersToRemove)
+    public static string? RemoveCharacters(this string? input, char[]? charactersToRemove)
     {
         if (string.IsNullOrEmpty(input) || charactersToRemove is null || charactersToRemove.Length == 0)
         {
@@ -71,7 +71,8 @@ public static string RemoveCharacters(this ReadOnlySpan<char> input, ReadOnlySpa
     /// <param name="input">The input string.</param>
     /// <param name="substring">The substring to count.</param>
     /// <returns>The number of occurrences of the substring in the input string.</returns>
-    public static int CountSubstring(this string input, string substring)
+    /// <remarks>Returns <c>0</c> if <paramref name="input"/> or <paramref name="substring"/> is <see langword="null"/>.</remarks>
+    public static int CountSubstring(this string? input, string? substring)
     {
         return CountSubstring(input.AsSpan(), substring.AsSpan());
     }
@@ -99,7 +100,7 @@ public static int CountSubstring(this ReadOnlySpan<char> input, ReadOnlySpan<cha
     /// <param name="input">The input to reverse words.</param>
     /// <returns>The input string with the order of words reversed.</returns>
     /// <remarks>Returns <see langword="null"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
-    public static string ReverseWords(this string input)
+    public static string? ReverseWords(this string? input)
     {
         if (string.IsNullOrEmpty(input))
         {
@@ -133,7 +134,7 @@ public static string ReverseWords(this string input)
     /// <param name="input">The input string.</param>
     /// <returns>A new string with duplicate characters removed.</returns>
     /// <remarks>Returns <see langword="null"/> if <paramref name="input"/> is <see langword="null"/>.</remarks>
-    public static string RemoveDuplicateCharacters(this string input)
+    public static string? RemoveDuplicateCharacters(this string? input)
     {
         if (string.IsNullOrEmpty(input))
         {
diff --git a/StringExtension/Validation/Validation.cs b/StringExtension/Validation/Validation.cs
index 814f603..acbb74d 100644
--- a/StringExtension/Validation/Validation.cs
+++ b/StringExtension/Validation/Validation.cs
@@ -26,7 +26,8 @@ public static partial class Validation
     /// </summary>
     /// <param name="email">The email address to validate.</param>
     /// <returns><c>true</c> if the given email address is valid; otherwise, <c>false</c>.</returns>
-    public static bool IsValidEmail(this string email)
+    /// <remarks>Returns <see langword="false"/> if <paramref name="email"/> is <see langword="null"/>.</remarks>
+    public static bool IsValidEmail(this string? email)
     {
         return !string.IsNullOrEmpty(email) && MailAddressRegex().IsMatch(email);
     }
@@ -46,7 +47,8 @@ public static bool IsValidEmail(this ReadOnlySpan<char> email)
     /// </summary>
     /// <param name="phoneNumber">The phone number to validate.</param>
     /// <returns><c>true</c> if the given phone number is valid; otherwise, <c>false</c>.</returns>
-    public static bool IsValidPhoneNumber(this string phoneNumber)
+    /// <remarks>Returns <see langword="false"/> if <paramref name="phoneNumber"/> is <see langword="null"/>.</remarks>
+    public static bool IsValidPhoneNumber(this string? phoneNumber)
     {
         return !string.IsNullOrEmpty(phoneNumber) && PhoneNumberRegex().IsMatch(phoneNumber);
     }
diff --git a/UnitTests/StringExtensionTests.cs b/UnitTests/StringExtensionTests.cs
index dc53768..a9271a3 100644
--- a/UnitTests/StringExtensionTests.cs
+++ b/UnitTests/StringExtensionTests.cs
@@ -19,7 +19,7 @@ public void TestRemoveCharacters()
         string input = "hello world!";
         char[] charactersToRemove = { 'l', 'o' };
         string expected = "he wrd!";
-        string result = input.RemoveCharacters(charactersToRemove);
+        string? result = input.RemoveCharacters(charactersToRemove);
         Assert.That(result, Is.EqualTo(expected));
     }
 
@@ -31,7 +31,7 @@ public void TestRemoveCharacters_NullInput()
     {
         string input = null!;
         char[] charactersToRemove = { 'l', 'o' };
-        string result = input.RemoveCharacters(charactersToRemove);
+        string? result = input.RemoveCharacters(charactersToRemove);
         Assert.That(result, Is.Null);
     }
 
@@ -91,7 +91,7 @@ public void TestReverseWords()
     {
         string input = "hello world!";
         string expected = "world! hello";
-        string result = input.ReverseWords();
+        string? result = input.ReverseWords();
         Assert.That(result, Is.EqualTo(expected));
     }
 
@@ -102,7 +102,7 @@ public void TestReverseWords()
     public void TestReverseWords_NullInput()
     {
         string input = null!;
-        string result = input.ReverseWords();
+        string? result = input.ReverseWords();
         Assert.That(result, Is.Null);
     }
 
@@ -190,7 +190,7 @@ public void TestRemoveDuplicateCharacters()
     {
         string input = "hello world!";
         string expected = "helo wrd!";
-        string result = input.RemoveDuplicateCharacters();
+        string? result = input.RemoveDuplicateCharacters();
         Assert.That(result, Is.EqualTo(expected));
     }
 
@@ -330,7 +330,6 @@ public void TestToTitleCase_LastWordAlwaysCapitalized()
         Assert.That(result, Is.EqualTo(expected));
     }
 
-
     /// <summary>
     /// Tests the Slugify method with accented characters and punctuation.
     /// </summary>