diff --git a/gtk/main.c b/gtk/main.c
index 354b8bfe0..8498aa695 100644
--- a/gtk/main.c
+++ b/gtk/main.c
@@ -22,6 +22,7 @@
  * DEALINGS IN THE SOFTWARE.
  *****************************************************************************/
 
+#include <locale.h>
 #include <sys/param.h>
 #include <signal.h>
 #include <string.h>