diff --git a/src/osgViewer/CMakeLists.txt b/src/osgViewer/CMakeLists.txt
index fc6164faf..614e29879 100644
--- a/src/osgViewer/CMakeLists.txt
+++ b/src/osgViewer/CMakeLists.txt
@@ -82,9 +82,9 @@ IF(WIN32 AND NOT ANDROID)
     )
 ELSE()
     IF(ANDROID)
-      SET(OSG_WINDOWING_SYSTEM "None" CACHE STRING "None Windowing system type for graphics window creation.")
+      SET(OSG_WINDOWING_SYSTEM "None" CACHE STRING "None Windowing system type for graphics window creation." FORCE)
     ELSE()
-      SET(OSG_WINDOWING_SYSTEM "X11" CACHE STRING "Windowing system type for graphics window creation. options only X11")
+      SET(OSG_WINDOWING_SYSTEM "X11" CACHE STRING "Windowing system type for graphics window creation. options only X11" FORCE)
     ENDIF()
 
     IF(${OSG_WINDOWING_SYSTEM} STREQUAL "Cocoa")