diff --git a/lib/sl/sl_locl.h b/lib/sl/sl_locl.h
index 0cb1bd4dc..d777e4de0 100644
--- a/lib/sl/sl_locl.h
+++ b/lib/sl/sl_locl.h
@@ -40,6 +40,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <stdarg.h>
+#include <ctype.h>
 
 #include <roken.h>