diff --git a/src/libsodium/randombytes/randombytes.c b/src/libsodium/randombytes/randombytes.c
index 929180b7..abcffb88 100644
--- a/src/libsodium/randombytes/randombytes.c
+++ b/src/libsodium/randombytes/randombytes.c
@@ -136,10 +136,12 @@ randombytes_uniform(const uint32_t upper_bound)
     if (upper_bound < 2) {
         return 0;
     }
-    min = (1U + ~upper_bound) % upper_bound;
+    min = (1U + ~upper_bound) % upper_bound; /* = 2**32 mod upper_bound */
     do {
         r = randombytes_random();
     } while (r < min);
+    /* r is now clamped to a set whose size mod upper_bound == 0
+     * the worst case (2**31+1) requires ~ 2 attempts */
 
     return r % upper_bound;
 }