diff --git a/lib/redmine/export/csv.rb b/lib/redmine/export/csv.rb
index dfbe36f4f..3b59ae30f 100644
--- a/lib/redmine/export/csv.rb
+++ b/lib/redmine/export/csv.rb
@@ -29,19 +29,18 @@ module Redmine
       class Base < ::CSV
         include Redmine::I18n
 
+        BOM = "\xEF\xBB\xBF".force_encoding('UTF-8')
         class << self
 
           def generate(&block)
             col_sep = l(:general_csv_separator)
             encoding = l(:general_csv_encoding)
 
-            str = ''.force_encoding(encoding)
-            if encoding == 'UTF-8'
-              # BOM
-              str = "\xEF\xBB\xBF".force_encoding(encoding)
+            res = super(:col_sep => col_sep, :encoding => encoding, &block)
+            if encoding == 'UTF-8' && !res.starts_with?(BOM)
+              res = BOM + res
             end
-
-            super(str, :col_sep => col_sep, :encoding => encoding, &block)
+            return res
           end
         end