diff --git a/news/changelog-1.9.md b/news/changelog-1.9.md
index e409e9a3b33..be96d684a8c 100644
--- a/news/changelog-1.9.md
+++ b/news/changelog-1.9.md
@@ -6,6 +6,7 @@ All changes included in 1.9:
 - ([#13441](https://github.com/quarto-dev/quarto-cli/pull/13441)): Catch `undefined` exceptions in Pandoc failure to avoid spurious error message.
 - ([#13046](https://github.com/quarto-dev/quarto-cli/issues/13046)): Use new url for multiplex socket.io server <https://multiplex.up.railway.app/> as default for `format: revealjs` and `revealjs.multiplex: true`.
 - ([#13506](https://github.com/quarto-dev/quarto-cli/issues/13506)): Fix navbar active state detection when sidebar has no logo configured. Prevents empty logo links from interfering with navigation highlighting.
+- ([#13616](https://github.com/quarto-dev/quarto-cli/issues/13616)): Fix fatal error when rendering manuscript projects with custom blocks like ConditionalBlock.
 - ([#13625](https://github.com/quarto-dev/quarto-cli/issues/13625)): Fix Windows file locking error (os error 32) when rendering with `--output-dir` flag. Context cleanup now happens before removing the temporary `.quarto` directory, ensuring file handles are properly closed.
 - ([#13633](https://github.com/quarto-dev/quarto-cli/issues/13633)): Fix detection and auto-installation of babel language packages from newer error format that doesn't explicitly mention `.ldf` filename.
 
diff --git a/src/resources/filters/layout/manuscript.lua b/src/resources/filters/layout/manuscript.lua
index a0a3f18cc40..ab21d42e138 100644
--- a/src/resources/filters/layout/manuscript.lua
+++ b/src/resources/filters/layout/manuscript.lua
@@ -57,10 +57,10 @@ function manuscript()
         -- If this is a notebook embed cell, 'lift' the contents of any child divs
         -- up (unroll their contents), this will help us avoid
         -- labeling divs marked as `cells` more than once
-        local blocks = pandoc.List()
+        local blocks = pandoc.Blocks({})
         for _, childBlock in ipairs(divEl.content) do
           if is_regular_node(childBlock, "Div") then
-              tappend(blocks, childBlock.content)
+              blocks:extend(childBlock.content)
           else
             blocks:insert(childBlock)
           end
diff --git a/tests/docs/smoke-all/2025/10/27/13616/.gitignore b/tests/docs/smoke-all/2025/10/27/13616/.gitignore
new file mode 100644
index 00000000000..ad293093b07
--- /dev/null
+++ b/tests/docs/smoke-all/2025/10/27/13616/.gitignore
@@ -0,0 +1,2 @@
+/.quarto/
+**/*.quarto_ipynb
diff --git a/tests/docs/smoke-all/2025/10/27/13616/13616.qmd b/tests/docs/smoke-all/2025/10/27/13616/13616.qmd
new file mode 100644
index 00000000000..85e7938dae9
--- /dev/null
+++ b/tests/docs/smoke-all/2025/10/27/13616/13616.qmd
@@ -0,0 +1,13 @@
+---
+title: "Conditional Block in manuscript article"
+_quarto:
+  tests:
+    pdf:
+      noErrors: true
+---
+
+::: {.content-visible when-format="pdf"}
+
+Something
+
+:::
diff --git a/tests/docs/smoke-all/2025/10/27/13616/_quarto.yml b/tests/docs/smoke-all/2025/10/27/13616/_quarto.yml
new file mode 100644
index 00000000000..2539ccd82a5
--- /dev/null
+++ b/tests/docs/smoke-all/2025/10/27/13616/_quarto.yml
@@ -0,0 +1,9 @@
+project:
+  type: manuscript
+
+manuscript:
+  article: 13616.qmd
+
+format:
+  pdf:
+    pdf-engine: xelatex