diff --git a/crates/typst-pdf/src/tags/context.rs b/crates/typst-pdf/src/tags/context.rs
index 8011f1cb6..c9ecb0997 100644
--- a/crates/typst-pdf/src/tags/context.rs
+++ b/crates/typst-pdf/src/tags/context.rs
@@ -75,6 +75,8 @@ impl Tags {
     }
 
     pub fn build_tree(&mut self) -> TagTree {
+        assert!(self.stack.items.is_empty(), "tags weren't properly closed");
+
         let children = std::mem::take(&mut self.tree)
             .into_iter()
             .map(|node| self.resolve_node(node))