diff --git a/include/osgUtil/CullVisitor b/include/osgUtil/CullVisitor
index 3b6140cac..26e9de050 100644
--- a/include/osgUtil/CullVisitor
+++ b/include/osgUtil/CullVisitor
@@ -175,14 +175,14 @@ class OSGUTIL_EXPORT CullVisitor : public osg::NodeVisitor, public osg::CullStac
 
         /** CullVisitor's default clamping of the projection float matrix to computed near and far values.
           * Note, do not call this method directly, use clampProjectionMatrix(..) instead, unless you want to bypass the callback.*/
-        virtual bool clampProjectionMatrixImplementation(osg::Matrixf& projection, double znear, double zfar) const;
+        virtual bool clampProjectionMatrixImplementation(osg::Matrixf& projection, double& znear, double& zfar) const;
 
         /** CullVisitor's default clamping of the projection double matrix to computed near and far values.
           * Note, do not call this method directly, use clampProjectionMatrix(..) instead, unless you want to bypass the callback.*/
-        virtual bool clampProjectionMatrixImplementation(osg::Matrixd& projection, double znear, double zfar) const;
+        virtual bool clampProjectionMatrixImplementation(osg::Matrixd& projection, double& znear, double& zfar) const;
 
         /** clamp the projection float matrix to computed near and far values, use callback if it exists, otherwise use default CullVisitro implemntation.*/
-        inline bool clampProjectionMatrix(osg::Matrixf& projection, value_type znear, value_type zfar) const
+        inline bool clampProjectionMatrix(osg::Matrixf& projection, value_type& znear, value_type& zfar) const
         {
             double zn = znear;
             double zf = zfar;