diff --git a/lib/asn1/lex.l b/lib/asn1/lex.l
index ac02d5cd5..b4b62989b 100644
--- a/lib/asn1/lex.l
+++ b/lib/asn1/lex.l
@@ -46,6 +46,9 @@
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 #include "symbol.h"
 #include "parse.h"