diff --git a/src/osgViewer/CMakeLists.txt b/src/osgViewer/CMakeLists.txt
index 2ba2cd1dd..f32e0ee1a 100644
--- a/src/osgViewer/CMakeLists.txt
+++ b/src/osgViewer/CMakeLists.txt
@@ -160,7 +160,7 @@ ELSE()
 
         SET(LIB_EXTRA_LIBS ${COCOA_LIBRARY} ${LIB_EXTRA_LIBS})
 
-    ELSEIF(${OSG_WINDOWING_SYSTEM} STREQUAL "x11")
+    ELSEIF(${OSG_WINDOWING_SYSTEM} STREQUAL "X11")
         # X11 for everybody else
         INCLUDE(FindPkgConfig OPTIONAL)
         IF(PKG_CONFIG_FOUND)