diff --git a/sphinx/search/de.py b/sphinx/search/de.py
index 89cbe3de5..35c6263c4 100644
--- a/sphinx/search/de.py
+++ b/sphinx/search/de.py
@@ -311,4 +311,5 @@ class SearchGerman(SearchLanguage):
         self.stemmer = snowballstemmer.stemmer('german')
 
     def stem(self, word):
+        word = word.lower()
         return self.stemmer.stemWord(word)