diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index b05b1ee6..135eedee 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -27,6 +27,7 @@ extern "C" {
 #endif
 #if SEXP_USE_GREEN_THREADS || SEXP_USE_GC_FILE_DESCRIPTORS
 #include <errno.h>
+#include <unistd.h>
 #endif
 #if SEXP_USE_GREEN_THREADS
 #include <sys/time.h>