diff --git a/libc/include/sys/user.h b/libc/include/sys/user.h
index 0e36825..b370add 100644
--- a/libc/include/sys/user.h
+++ b/libc/include/sys/user.h
@@ -31,6 +31,7 @@
 
 #include <sys/cdefs.h>
 #include <limits.h> /* For PAGE_SIZE. */
+#include <stddef.h> /* For size_t. */
 
 __BEGIN_DECLS