diff --git a/lib/redmine/export/pdf.rb b/lib/redmine/export/pdf.rb
index e821c4e8a..bc49b8c75 100644
--- a/lib/redmine/export/pdf.rb
+++ b/lib/redmine/export/pdf.rb
@@ -422,7 +422,7 @@ module Redmine
           if query.grouped? &&
                (group = query.group_by_column.value(issue)) != previous_group
             pdf.SetFontStyle('B',10)
-            group_label = group.blank? ? 'None' : group.to_s
+            group_label = group.blank? ? 'None' : group.to_s.dup
             group_label << " (#{query.issue_count_by_group[group]})"
             pdf.Bookmark group_label, 0, -1
             pdf.RDMCell(table_width + col_id_width, row_height * 2, group_label, 1, 1, 'L')