diff --git a/third_party/zlib/gzguts.h b/third_party/zlib/gzguts.h
index 57faf37165..54f668fcb2 100644
--- a/third_party/zlib/gzguts.h
+++ b/third_party/zlib/gzguts.h
@@ -3,6 +3,10 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#ifndef _WIN32
+  #include <unistd.h>
+#endif
+
 #ifdef _LARGEFILE64_SOURCE
 #  ifndef _LARGEFILE_SOURCE
 #    define _LARGEFILE_SOURCE 1