diff --git a/src/Fl_x.cxx b/src/Fl_x.cxx
index 51570d9f5..913c7200d 100644
--- a/src/Fl_x.cxx
+++ b/src/Fl_x.cxx
@@ -51,6 +51,7 @@
 #  include <X11/Xmd.h>
 #  include <X11/Xlocale.h>
 #  include <X11/Xlib.h>
+#  include <X11/keysym.h>
 
 static Fl_Xlib_Graphics_Driver fl_xlib_driver;
 static Fl_Display_Device fl_xlib_display(&fl_xlib_driver);