diff --git a/components/shader/shadermanager.hpp b/components/shader/shadermanager.hpp
index 8aa299c73b..50c430637e 100644
--- a/components/shader/shadermanager.hpp
+++ b/components/shader/shadermanager.hpp
@@ -13,6 +13,10 @@
 #include <osg/Shader>
 #include <osg/Program>
 
+namespace osgViewer {
+    class Viewer;
+}
+
 namespace Shader
 {
     struct HotReloadManager;