diff --git a/scrapy/core/downloader/handlers/http11.py b/scrapy/core/downloader/handlers/http11.py
index c7553eb87..a0fd837b1 100644
--- a/scrapy/core/downloader/handlers/http11.py
+++ b/scrapy/core/downloader/handlers/http11.py
@@ -437,7 +437,7 @@ class ScrapyAgent:
         try:
             version = result["txresponse"].version
             protocol = f"{to_unicode(version[0])}/{version[1]}.{version[2]}"
-        except (AttributeError, TypeError):
+        except (AttributeError, TypeError, IndexError):
             protocol = None
         response = respcls(
             url=url,