diff --git a/src/call_out.c b/src/call_out.c
index e62014c6f296acf1495afb14b5ee0df15b5383c4..7ed40e90fad31e074798021fbd7c8c841d426dba 100644
--- a/src/call_out.c
+++ b/src/call_out.c
@@ -7,6 +7,7 @@
 #include "interpret.h"
 #include "error.h"
 #include "builtin_efuns.h"
+#include "sys/types.h"
 
 call_out **pending_calls=0;      /* pointer to first busy pointer */
 int num_pending_calls;           /* no of busy pointers in buffer */