diff --git a/libc/include/bits/swab.h b/libc/include/bits/swab.h
index 63281b6e0..9591c2ede 100644
--- a/libc/include/bits/swab.h
+++ b/libc/include/bits/swab.h
@@ -38,7 +38,7 @@
 
 __BEGIN_DECLS
 
-__BIONIC_SWAB_INLINE void swab(const void* __void_src, void* __void_dst, ssize_t __byte_count) {
+__BIONIC_SWAB_INLINE void swab(const void* _Nonnull __void_src, void* _Nonnull __void_dst, ssize_t __byte_count) {
   const uint8_t* __src = __BIONIC_CAST(static_cast, const uint8_t*, __void_src);
   uint8_t* __dst = __BIONIC_CAST(static_cast, uint8_t*, __void_dst);
   while (__byte_count > 1) {