1 files changed, 5 insertions, 5 deletions
diff --git a/lib/axlsx/drawing/num_data_source.rb b/lib/axlsx/drawing/num_data_source.rb
index c52b0fa8..e0e97e41 100644
--- a/lib/axlsx/drawing/num_data_source.rb
+++ b/lib/axlsx/drawing/num_data_source.rb
@@ -46,16 +46,16 @@ module Axlsx
     # serialize the object
     # @param [String] str
     def to_xml_string(str="")
-      str << '<c:' << tag_name.to_s << '>'
+      str << ('<c:' << tag_name.to_s << '>')
       if @f
-        str << '<c:' << @ref_tag_name.to_s << '>'
-        str << '<c:f>' << @f.to_s << '</c:f>'
+        str << ('<c:' << @ref_tag_name.to_s << '>')
+        str << ('<c:f>' << @f.to_s << '</c:f>')
       end
       @data.to_xml_string str
       if @f
-        str << '</c:' << @ref_tag_name.to_s << '>'
+        str << ('</c:' << @ref_tag_name.to_s << '>')
       end
-      str << '</c:' << tag_name.to_s << '>'
+      str << ('</c:' << tag_name.to_s << '>')
     end
   end
 end