diff --git a/EssentialCSharp.Web.Tests/SiteMappingTests.cs b/EssentialCSharp.Web.Tests/SiteMappingTests.cs
index 91b4e7d9..457b2e9d 100644
--- a/EssentialCSharp.Web.Tests/SiteMappingTests.cs
+++ b/EssentialCSharp.Web.Tests/SiteMappingTests.cs
@@ -59,6 +59,13 @@ public async Task FindHelloWorldWithAnchorSlugReturnsCorrectSiteMap()
         await Assert.That(foundSiteMap).IsEquivalentTo(HelloWorldSiteMapping);
     }
 
+    [Test]
+    public async Task Find_NullSiteMappings_ThrowsArgumentNullException()
+    {
+        await Assert.That(() => ((IList<SiteMapping>)null!).Find("hello-world"))
+            .Throws<ArgumentNullException>();
+    }
+
     [Test]
     public async Task FindCSyntaxFundamentalsWithSpacesReturnsCorrectSiteMap()
     {
@@ -95,6 +102,13 @@ public async Task FindPercentComplete_KeyIsNull_ReturnsNull()
         await Assert.That(percent).IsNull();
     }
 
+    [Test]
+    public async Task FindPercentComplete_NullSiteMappings_ThrowsArgumentNullException()
+    {
+        await Assert.That(() => ((IList<SiteMapping>)null!).FindPercentComplete("hello-world"))
+            .Throws<ArgumentNullException>();
+    }
+
     [Test]
     [Arguments("   ")]
     [Arguments("")]
diff --git a/EssentialCSharp.Web/Extensions/SiteMappingListExtensions.cs b/EssentialCSharp.Web/Extensions/SiteMappingListExtensions.cs
index 3e50d4e5..90ba2e37 100644
--- a/EssentialCSharp.Web/Extensions/SiteMappingListExtensions.cs
+++ b/EssentialCSharp.Web/Extensions/SiteMappingListExtensions.cs
@@ -12,6 +12,8 @@ public static class SiteMappingListExtensions
     /// <returns>If found, the site mapping that matches the key, otherwise null.</returns>
     public static SiteMapping? Find(this IList<SiteMapping> siteMappings, string? key)
     {
+        ArgumentNullException.ThrowIfNull(siteMappings);
+
         if (string.IsNullOrWhiteSpace(key))
         {
             return siteMappings.FirstOrDefault();
@@ -28,6 +30,8 @@ public static class SiteMappingListExtensions
     /// <returns>Returns a formatted double for use as the percent complete.</returns>
     public static string? FindPercentComplete(this IList<SiteMapping> siteMappings, string? key)
     {
+        ArgumentNullException.ThrowIfNull(siteMappings);
+
         if (key is null)
         {
             return null;