diff --git a/include/mapnik/xml_tree.hpp b/include/mapnik/xml_tree.hpp
index 25bd00d24..61414d759 100644
--- a/include/mapnik/xml_tree.hpp
+++ b/include/mapnik/xml_tree.hpp
@@ -43,7 +43,7 @@ class xml_tree
 {
 public:
     xml_tree(std::string const& encoding="utf8");
-    void set_filename(std::string fn);
+    void set_filename(std::string const& fn);
     std::string const& filename() const;
     xml_node &root();
     xml_node const& root() const;
diff --git a/src/xml_tree.cpp b/src/xml_tree.cpp
index af04dcc9d..4e590b77b 100644
--- a/src/xml_tree.cpp
+++ b/src/xml_tree.cpp
@@ -106,7 +106,7 @@ xml_tree::xml_tree(std::string const& encoding)
     node_.set_processed(true); //root node is always processed
 }
 
-void xml_tree::set_filename(std::string fn)
+void xml_tree::set_filename(std::string const& fn)
 {
     file_ = fn;
 }