diff --git a/util/random.h b/util/random.h
index 400141a18..99879dca8 100644
--- a/util/random.h
+++ b/util/random.h
@@ -78,7 +78,7 @@ long int ub_random(struct ub_randstate* state);
 /**
  * Generate random number between 0 and x-1.  No modulo bias.
  * @param state: must have been initialised with ub_initstate.
- * @param x: an upper limit. not negative or zero. must be smaller than 2**31.
+ * @param x: an upper limit. not (negative or zero). must be smaller than 2**31.
  * @return: random value between 0..x-1. Possibly more than one
  * random number is picked from the random stream to satisfy this.
  */