diff --git a/lib/ruby-tf-idf.rb b/lib/ruby-tf-idf.rb
index 9b124de..7929c85 100644
--- a/lib/ruby-tf-idf.rb
+++ b/lib/ruby-tf-idf.rb
@@ -97,7 +97,7 @@ def split_docs(docs)
       splitted_docs = []
       docs.each do |d|
         begin
-          splitted_docs << d.downcase!.gsub(/,|\.|\'/,'').split(/\s+/)
+          splitted_docs << d.downcase.gsub(/,|\.|\'/,'').split(/\s+/)
         rescue
         end
       end