diff --git a/gensim/summarization/bm25.py b/gensim/summarization/bm25.py
index b287cdd48d..6cf7e8a043 100644
--- a/gensim/summarization/bm25.py
+++ b/gensim/summarization/bm25.py
@@ -85,7 +85,7 @@ def initialize(self):
         """Calculates frequencies of terms in documents and in corpus. Also computes inverse document frequencies."""
         for document in self.corpus:
             frequencies = {}
-            doc_length.append(len(document))
+            (self.doc_length).append(len(document))
             for word in document:
                 if word not in frequencies:
                     frequencies[word] = 0