diff --git a/cdefs.h b/cdefs.h
index cb25145..7b8ed5b 100644
--- a/cdefs.h
+++ b/cdefs.h
@@ -164,13 +164,13 @@
    || (__builtin_constant_p (__l) && (__l) > 0))
 
 /* Length is known to be safe at compile time if the __L * __S <= __OBJSZ
-   condition can be folded to a constant and if it is true.  The -1 check is
-   redundant because since it implies that __glibc_safe_len_cond is true.  */
+   condition can be folded to a constant and if it is true, or unknown (-1) */
 #define __glibc_safe_or_unknown_len(__l, __s, __osz) \
-  (__glibc_unsigned_or_positive (__l)					      \
-   && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l),     \
-						   __s, __osz))		      \
-   && __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), __s, __osz))
+  ((__osz) == (__SIZE_TYPE__) -1					      \
+   || (__glibc_unsigned_or_positive (__l)				      \
+       && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \
+						       (__s), (__osz)))	      \
+       && __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), (__s), (__osz))))
 
 /* Conversely, we know at compile time that the length is unsafe if the
    __L * __S <= __OBJSZ condition can be folded to a constant and if it is