diff --git a/ext/dl/dl.c b/ext/dl/dl.c
index 6b7f01b3ff..073d8180d2 100644
--- a/ext/dl/dl.c
+++ b/ext/dl/dl.c
@@ -4,6 +4,7 @@
 
 #include <ruby.h>
 #include <rubyio.h>
+#include <ctype.h>
 #include "dl.h"
 
 VALUE rb_mDL;