diff --git a/EssentialCSharp.Web.Tests/FunctionalTests.cs b/EssentialCSharp.Web.Tests/FunctionalTests.cs
index 31aa723e..005f27f7 100644
--- a/EssentialCSharp.Web.Tests/FunctionalTests.cs
+++ b/EssentialCSharp.Web.Tests/FunctionalTests.cs
@@ -44,6 +44,19 @@ public async Task WhenPagesAreAccessed_TheyReturnHtml(string relativeUrl)
         await Assert.That(content).Contains("<html", StringComparison.OrdinalIgnoreCase);
     }
 
+    [Test]
+    [Arguments("/guidelines", "window.PERCENT_COMPLETE = null;")]
+    public async Task WhenNonContentPageIsRendered_LayoutIncludesNullPercentComplete(string relativeUrl, string expectedSnippet)
+    {
+        HttpClient client = factory.CreateClient();
+        using HttpResponseMessage response = await client.GetAsync(relativeUrl);
+
+        await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.OK);
+
+        string content = await response.Content.ReadAsStringAsync();
+        await Assert.That(content).Contains(expectedSnippet);
+    }
+
     [Test]
     public async Task WhenTheApplicationStarts_NonExistingPage_GivesCorrectStatusCode()
     {
@@ -52,4 +65,4 @@ public async Task WhenTheApplicationStarts_NonExistingPage_GivesCorrectStatusCod
 
         await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.NotFound);
     }
-}
\ No newline at end of file
+}
diff --git a/EssentialCSharp.Web/src/composables/useSiteShell.js b/EssentialCSharp.Web/src/composables/useSiteShell.js
index bcc87ca7..cc41a379 100644
--- a/EssentialCSharp.Web/src/composables/useSiteShell.js
+++ b/EssentialCSharp.Web/src/composables/useSiteShell.js
@@ -2,6 +2,7 @@ import { computed, nextTick, onMounted, onUnmounted, reactive, ref, watch } from
 import { useWindowSize } from "./useWindowSize.js";
 
 const SMALL_SCREEN_SIZE = 768;
+const NON_CONTENT_PERCENT_COMPLETE_VALUES = [null, "0.00"];
 
 /**
  * Find the path of TOC entries that lead to the current page.
@@ -71,7 +72,7 @@ export function useSiteShell() {
     const smallScreen = computed(() => (windowWidth.value || 0) < SMALL_SCREEN_SIZE);
     const currentPage = findCurrentPage([], tocData) ?? [];
     const chapterParentPage = currentPage.find((parent) => parent.level === 0) ?? null;
-    const isContentPage = computed(() => percentComplete.value !== null);
+    const isContentPage = computed(() => !NON_CONTENT_PERCENT_COMPLETE_VALUES.includes(percentComplete.value));
 
     for (const item of currentPage) {
         expandedTocs.add(item.key);