diff --git a/xmodmap.c b/xmodmap.c
index 58a8e70..0aafed5 100644
--- a/xmodmap.c
+++ b/xmodmap.c
@@ -34,6 +34,8 @@ from The Open Group.
 #include <stdarg.h>
 #include "xmodmap.h"
 
+extern int asprintf (char **, const char *, ...);
+
 const char *ProgramName;
 Display *dpy = NULL;
 int min_keycode, max_keycode;