diff --git a/src/drivers/Posix/Fl_Posix_System_Driver.cxx b/src/drivers/Posix/Fl_Posix_System_Driver.cxx
index b3d6f0bad..98897e41e 100644
--- a/src/drivers/Posix/Fl_Posix_System_Driver.cxx
+++ b/src/drivers/Posix/Fl_Posix_System_Driver.cxx
@@ -18,6 +18,7 @@
 
 #include <config.h>
 #include "Fl_Posix_System_Driver.H"
+#include "../../flstring.h"
 #include <FL/filename.H>
 #include <FL/Fl.H>
 #include <X11/Xlib.h>