diff --git a/ext/zlib/gzguts.h b/ext/zlib/gzguts.h
index d87659d031..cbd680da48 100644
--- a/ext/zlib/gzguts.h
+++ b/ext/zlib/gzguts.h
@@ -27,6 +27,10 @@
 #endif
 #include <fcntl.h>
 
+#ifdef __APPLE__
+#include <unistd.h>
+#endif
+
 #ifdef _WIN32
 #  include <stddef.h>
 #endif