diff --git a/src/osgFX/SpecularHighlights.cpp b/src/osgFX/SpecularHighlights.cpp
index 2baba5b29..5b4092d73 100644
--- a/src/osgFX/SpecularHighlights.cpp
+++ b/src/osgFX/SpecularHighlights.cpp
@@ -105,7 +105,7 @@ namespace
         {
         }
 
-        virtual void getRequiredExtensions(std::vector<std::string>& extensions)
+        virtual void getRequiredExtensions(std::vector<std::string>& extensions) const
         {
             extensions.push_back("GL_ARB_texture_env_add");
         }