diff --git a/routes/document_helpers.py b/routes/document_helpers.py
index ebfb177..134157e 100644
--- a/routes/document_helpers.py
+++ b/routes/document_helpers.py
@@ -203,6 +203,8 @@ def _assert_pdf_marker_upload_owned(
 def _derive_title(content: str) -> str:
     """Derive a title from document content."""
     import re
+    if not isinstance(content, str):
+        return "Untitled"
     text = content.strip()
     if not text:
         return "Untitled"
diff --git a/tests/test_derive_title_nonstring.py b/tests/test_derive_title_nonstring.py
new file mode 100644
index 0000000..c5b75c7
--- /dev/null
+++ b/tests/test_derive_title_nonstring.py
@@ -0,0 +1,13 @@
+from routes.document_helpers import _derive_title
+
+
+def test_derive_title_handles_non_string_content():
+    # content normally comes from a document text column, but the helper is
+    # public and a non-string (None / int) made content.strip() raise
+    # AttributeError instead of falling back to a default title.
+    assert _derive_title(None) == "Untitled"
+    assert _derive_title(123) == "Untitled"
+
+
+def test_derive_title_still_reads_markdown_heading():
+    assert _derive_title("# Heading Title\nbody text") == "Heading Title"