diff --git a/spacy/lemmatizer.py b/spacy/lemmatizer.py
index a070574bb..1f0f0da3f 100644
--- a/spacy/lemmatizer.py
+++ b/spacy/lemmatizer.py
@@ -56,6 +56,11 @@ class Lemmatizer(object):
         index_table = self.lookups.get_table("lemma_index", {})
         exc_table = self.lookups.get_table("lemma_exc", {})
         rules_table = self.lookups.get_table("lemma_rules", {})
+        if not any((index_table.get(univ_pos), exc_table.get(univ_pos), rules_table.get(univ_pos))):
+            if univ_pos == "propn":
+                return [string]
+            else:
+                return [string.lower()]
         lemmas = self.lemmatize(
             string,
             index_table.get(univ_pos, {}),