--- a/include/dos.h
+++ b/include/dos.h
@@ -26,6 +26,7 @@
 #include <net/ppp_defs.h>
 #include <sys/ioctl.h>
 #include <time.h>
+#include <ctype.h> /* isdigit() */
 
 /*
 #define DEBUG_TREE	1