diff --git a/library/bignum.c b/library/bignum.c
index e555692..110c3f4 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1301,7 +1301,7 @@
             r0 -= m;
 
             Z.p[i - t - 1] = ( q1 << biH ) | q0;
-#endif
+#endif /* POLARSSL_HAVE_UDBL && !64-bit Apple with Clang 5.0 */
         }
 
         Z.p[i - t - 1]++;