diff --git a/Modules/FindX11.cmake b/Modules/FindX11.cmake
index 01ba478ad..e12dc0a29 100644
--- a/Modules/FindX11.cmake
+++ b/Modules/FindX11.cmake
@@ -326,7 +326,9 @@ if (UNIX)
 
   # Most of the X11 headers will be in the same directories, avoid
   # creating a huge list of duplicates.
-  list(REMOVE_DUPLICATES X11_INCLUDE_DIR)
+  if (X11_INCLUDE_DIR)
+     list(REMOVE_DUPLICATES X11_INCLUDE_DIR)
+  endif ()
 
   # Deprecated variable for backwards compatibility with CMake 1.4
   if (X11_X11_INCLUDE_PATH AND X11_LIBRARIES)