diff --git a/doc/source/conf.py b/doc/source/conf.py
index 42ed94a..aaccedf 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -16,6 +16,7 @@ import os
 import subprocess
 import sys
 import time
+import warnings
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -137,6 +138,8 @@ try:
     html_last_updated_fmt = subprocess.Popen(
         git_cmd, stdout=subprocess.PIPE).communicate()[0]
 except Exception:
+    warnings.warn('Cannot get last updated time from git repository, '
+                  'using current time.')
     html_last_updated_fmt = time.ctime()
 
 # If true, SmartyPants will be used to convert quotes and dashes to