diff --git a/src/editfns.c b/src/editfns.c
index 71cb8fb744f..ffe52fe0b19 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -33,6 +33,8 @@ Boston, MA 02111-1307, USA.  */
 #include <unistd.h>
 #endif
 
+#include <ctype.h>
+
 #include "lisp.h"
 #include "intervals.h"
 #include "buffer.h"