diff --git a/porcupine/plugins/markdown.py b/porcupine/plugins/markdown.py
index 7e5b61ef2..01a1e8697 100644
--- a/porcupine/plugins/markdown.py
+++ b/porcupine/plugins/markdown.py
@@ -25,7 +25,13 @@ def _is_list_item(line: str) -> bool:
     - https://spec.commonmark.org/0.30/#lists
     - https://pandoc.org/MANUAL.html#lists
     """
+    assert isinstance(line, str)
+    if not line:
+        # empty string
+        return False
+
     assert len(line.splitlines()) == 1
+
     pattern = r"(^\s*\d{1,9}[.)]|^\s*[-+*]|^\s*#\)|^\s*#\.) .*"
     regex = re.compile(pattern)
     match = regex.search(line)