diff --git a/rank_bm25.py b/rank_bm25.py
index 31a0584..0008e38 100644
--- a/rank_bm25.py
+++ b/rank_bm25.py
@@ -93,7 +93,7 @@ def _calc_idf(self, nd):
         # idf can be negative if word is contained in more than half of documents
         negative_idfs = []
         for word, freq in nd.items():
-            idf = math.log(self.corpus_size - freq + 0.5) - math.log(freq + 0.5)
+            idf = math.log(self.corpus_size + 1) - math.log(freq + 0.5)
             self.idf[word] = idf
             idf_sum += idf
             if idf < 0: