diff --git a/src/utils.h b/src/utils.h
index bc90a11..ae68ab8 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -13,6 +13,11 @@
 
 #pragma warning(push, 0)
 #include <FL/fl_types.h>
+#ifndef _WIN32
+#  ifdef FLTK_USE_X11
+#  include <FL/x11.H>
+#  endif
+#endif
 #pragma warning(pop)
 
 #ifdef _WIN32