diff --git a/spec/rubyspec/optional/capi/ext/thread_spec.c b/spec/rubyspec/optional/capi/ext/thread_spec.c
index 5ea96fe3d6..78e8183129 100644
--- a/spec/rubyspec/optional/capi/ext/thread_spec.c
+++ b/spec/rubyspec/optional/capi/ext/thread_spec.c
@@ -4,7 +4,9 @@
 
 #include <math.h>
 #include <errno.h>
+#if HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 
 #ifdef __cplusplus
 extern "C" {