diff --git a/certbot-nginx/certbot_nginx/nginxparser.py b/certbot-nginx/certbot_nginx/nginxparser.py
index 412268d22..1de079cab 100644
--- a/certbot-nginx/certbot_nginx/nginxparser.py
+++ b/certbot-nginx/certbot_nginx/nginxparser.py
@@ -239,6 +239,8 @@ class UnspacedList(list):
         # Normalize indexes like list[-1] etc, and save the result
         if idx < 0:
             idx = len(self) + idx
+        if not 0 <= idx < len(self):
+            raise IndexError("list index out of range")
         idx0 = idx
         # Count the number of spaces in the spaced list before idx in the unspaced one
         while idx != -1: