diff --git a/idassigner.py b/idassigner.py
index 77400c2..8d7f4ca 100644
--- a/idassigner.py
+++ b/idassigner.py
@@ -11,6 +11,7 @@
 
 import argparse
 import logging
+import sys
 from datetime import datetime
 from lxml import etree
 from copy import deepcopy
@@ -24,7 +25,11 @@
 def assign_ids(chapter_tree):
     """Assign xml:ids to various elements"""
 
-    chapter_id = (chapter_tree.attrib["{http://www.w3.org/XML/1998/namespace}id"])
+    try:
+        chapter_id = (chapter_tree.attrib["{http://www.w3.org/XML/1998/namespace}id"])
+    except KeyError:
+        logging.error("Chapter has no id. Exiting")
+        sys.exit()
     logging.debug("The id of this chapter is %s", chapter_id)
 
     footnotes = chapter_tree.xpath("//t:note[@place='bottom']", namespaces=NS_MAP)