diff --git a/PDP10/pdp10_lp20.c b/PDP10/pdp10_lp20.c
index 64e501a8..3e0bde58 100644
--- a/PDP10/pdp10_lp20.c
+++ b/PDP10/pdp10_lp20.c
@@ -54,7 +54,7 @@
 */
 
 #include "pdp10_defs.h"
-#include "ctype.h"
+#include <ctype.h>
 
 /* Time (seconds) of idleness before data flushed to attached file. */
 #ifndef LP20_IDLE_TIME