diff --git a/X10D/src/Collections/DictionaryExtensions.cs b/X10D/src/Collections/DictionaryExtensions.cs
index 0c26126..c016654 100644
--- a/X10D/src/Collections/DictionaryExtensions.cs
+++ b/X10D/src/Collections/DictionaryExtensions.cs
@@ -1,4 +1,4 @@
-using System.Diagnostics.Contracts;
+using System.Diagnostics.Contracts;
 using System.Web;
 
 namespace X10D.Collections;
@@ -226,6 +226,11 @@ public static class DictionaryExtensions
             throw new ArgumentNullException(nameof(source));
         }
 
+        if (selector is null)
+        {
+            throw new ArgumentNullException(nameof(selector));
+        }
+
         static string SanitizeValue(string? value)
         {
             if (value is null)
@@ -268,6 +273,16 @@ public static class DictionaryExtensions
             throw new ArgumentNullException(nameof(source));
         }
 
+        if (keySelector is null)
+        {
+            throw new ArgumentNullException(nameof(keySelector));
+        }
+
+        if (valueSelector is null)
+        {
+            throw new ArgumentNullException(nameof(valueSelector));
+        }
+
         static string SanitizeValue(string? value)
         {
             if (value is null)
@@ -371,6 +386,11 @@ public static class DictionaryExtensions
             throw new ArgumentNullException(nameof(source));
         }
 
+        if (keySelector is null)
+        {
+            throw new ArgumentNullException(nameof(keySelector));
+        }
+
         if (valueSelector is null)
         {
             throw new ArgumentNullException(nameof(valueSelector));