diff --git a/lib/chibi/ast.c b/lib/chibi/ast.c
index 43b8c829..0b6fcc8e 100644
--- a/lib/chibi/ast.c
+++ b/lib/chibi/ast.c
@@ -3,6 +3,7 @@
 /*  BSD-style license: http://synthcode.com/license.txt       */
 
 #include <chibi/eval.h>
+#include <errno.h>
 
 #if ! SEXP_USE_BOEHM
 extern sexp sexp_gc (sexp ctx, size_t *sum_freed);