diff --git a/lib/axlsx/drawing/marker.rb b/lib/axlsx/drawing/marker.rb
index 75876fa3..cddc9b27 100644
--- a/lib/axlsx/drawing/marker.rb
+++ b/lib/axlsx/drawing/marker.rb
@@ -60,6 +60,7 @@ def to_xml_string(str = '')
       [:col, :colOff, :row, :rowOff].each do |k|
         str << ('<xdr:' << k.to_s << '>' << self.send(k).to_s << '</xdr:' << k.to_s << '>')
       end
+      str
     end
     private