diff --git a/src/osgPlugins/txp/trpage_io.h b/src/osgPlugins/txp/trpage_io.h
index 469614c07..2f123645d 100644
--- a/src/osgPlugins/txp/trpage_io.h
+++ b/src/osgPlugins/txp/trpage_io.h
@@ -395,7 +395,7 @@ public:
         */
     virtual void Add(const char *);
     // Same as const char * version
-    virtual void Add(std::string &);
+    virtual void Add(const std::string &);
     // Add a 32 bit float to the buffer
     virtual void Add(float32);
     // Add a 64 bit float to the buffer
diff --git a/src/osgPlugins/txp/trpage_writebuf.cpp b/src/osgPlugins/txp/trpage_writebuf.cpp
index d6e739bee..0c8248336 100644
--- a/src/osgPlugins/txp/trpage_writebuf.cpp
+++ b/src/osgPlugins/txp/trpage_writebuf.cpp
@@ -149,7 +149,7 @@ void trpgMemWriteBuffer::Add(const char *val)
 }
 
 // Add(std::string)
-void trpgMemWriteBuffer::Add(std::string &val)
+void trpgMemWriteBuffer::Add(const std::string &val)
 {
     Add(val.c_str());
 }