diff --git a/libc/include/string.h b/libc/include/string.h
index 56d89a4..02d8151 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -119,9 +119,16 @@
     return __builtin___strcpy_chk(dest, src, __builtin_object_size (dest, 0));
 }
 
+extern void __strncpy_error()
+    __attribute__((__error__("strncpy called with size bigger than buffer")));
+
 __BIONIC_FORTIFY_INLINE
 char *strncpy(char *dest, const char *src, size_t n) {
-    return __builtin___strncpy_chk(dest, src, n, __builtin_object_size (dest, 0));
+    size_t bos = __bos(dest);
+    if (__builtin_constant_p(n) && (n > bos)) {
+        __strncpy_error();
+    }
+    return __builtin___strncpy_chk(dest, src, n, bos);
 }
 
 __BIONIC_FORTIFY_INLINE
diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index 1976d6a..1288b28 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -517,6 +517,12 @@
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0 && !defined(__clang__)
 #define __BIONIC_FORTIFY 1
+#if _FORTIFY_SOURCE == 2
+#define __bos(s) __builtin_object_size((s), 1);
+#else
+#define __bos(s) __builtin_object_size((s), 0);
+#endif
+
 #define __BIONIC_FORTIFY_INLINE \
     extern inline \
     __attribute__ ((always_inline)) \