diff --git a/src/builtin_efuns.c b/src/builtin_efuns.c
index f8593a52c7120bd067c17758a5a22dd8e5873bf0..de64478002f9881a098052d7fcedda95b9d59d77 100644
--- a/src/builtin_efuns.c
+++ b/src/builtin_efuns.c
@@ -27,11 +27,13 @@
 #include "memory.h"
 #include "time_stuff.h"
 #include <math.h>
+#include <ctype.h>
 
 #ifdef HAVE_CRYPT_H
 #include <crypt.h>
 #endif
 
+
 void f_equal(INT32 args)
 {
   int i;