diff --git a/spacy/lang/lex_attrs.py b/spacy/lang/lex_attrs.py
index c3bb4a8ff..d9b84e666 100644
--- a/spacy/lang/lex_attrs.py
+++ b/spacy/lang/lex_attrs.py
@@ -136,7 +136,7 @@ def is_lower(string): return string.islower()
 def is_space(string): return string.isspace()
 def is_title(string): return string.istitle()
 def is_upper(string): return string.isupper()
-def is_stop(string, stops=set()): return string in stops
+def is_stop(string, stops=set()): return string.lower() in stops
 def is_oov(string): return True
 def get_prob(string): return -20.