diff --git a/src/OpenMesh/Core/IO/reader/PLYReader.cc b/src/OpenMesh/Core/IO/reader/PLYReader.cc
index db431c14..84d9fd78 100644
--- a/src/OpenMesh/Core/IO/reader/PLYReader.cc
+++ b/src/OpenMesh/Core/IO/reader/PLYReader.cc
@@ -54,6 +54,7 @@
 #include <fstream>
 #include <iostream>
 #include <memory>
+#include <string>
 
 #ifndef WIN32
 #endif
@@ -1089,7 +1090,7 @@ std::string get_property_name(std::string _string1, std::string _string2) {
 
 //-----------------------------------------------------------------------------
 
-_PLYReader_::ValueType get_property_type(std::string& _string1, std::string& _string2) {
+_PLYReader_::ValueType get_property_type(const std::string& _string1, const std::string& _string2) {
 
     if (_string1 == "float32" || _string2 == "float32")