diff --git a/src/lib/math/prob_distr.c b/src/lib/math/prob_distr.c
index f5e5218aa7..e170d000fe 100644
--- a/src/lib/math/prob_distr.c
+++ b/src/lib/math/prob_distr.c
@@ -1308,7 +1308,7 @@ sample_geometric(uint32_t s, double p0, double p)
   if (p >= 1)
     return 1;
 
-  return (-x/log1p(-p));
+  return ceil(-x/log1p(-p));
 }
 
 /*******************************************************************/
diff --git a/src/test/test_prob_distr.c b/src/test/test_prob_distr.c
index 75e7e360a6..ec4e943e9a 100644
--- a/src/test/test_prob_distr.c
+++ b/src/test/test_prob_distr.c
@@ -958,7 +958,17 @@ test_stochastic_geometric_impl(double p)
     size_t C[PSI_DF] = {0};
 
     for (j = 0; j < NSAMPLES; j++) {
-      double n_tmp = ceil(geometric_sample(p));
+      double n_tmp = geometric_sample(p);
+
+      /* Must be an integer.  (XXX -Wfloat-equal)  */
+      tor_assert(ceil(n_tmp) <= n_tmp && ceil(n_tmp) >= n_tmp);
+
+      /* Must be a positive integer.  */
+      tor_assert(n_tmp >= 1);
+
+      /* Probability of getting a value in the billions is negligible.  */
+      tor_assert(n_tmp <= (double)UINT_MAX);
+
       unsigned n = (unsigned) n_tmp;
 
       if (n > PSI_DF)