diff --git a/tests/testthat/test-helpers.R b/tests/testthat/test-helpers.R
index 601561a1..7d05e736 100644
--- a/tests/testthat/test-helpers.R
+++ b/tests/testthat/test-helpers.R
@@ -27,6 +27,14 @@ test_that("update_chain_stat works correctly", {
     ),
     stat_latest + pmin(1, n_offspring)
   )
+  expect_error(
+    .update_chain_stat(
+      stat_type = "foo",
+      stat_latest = stat_latest,
+      n_offspring = n_offspring
+    ),
+    "stat_type must be 'size' or 'length'"
+  )
 })
 
 test_that("get_statistic_func works correctly", {
@@ -38,4 +46,8 @@ test_that("get_statistic_func works correctly", {
     .get_statistic_func(chain_statistic = "length"),
     rgen_length
   )
+  expect_error(
+    .get_statistic_func(chain_statistic = "foo"),
+    "chain_statistic must be 'size' or 'length'"
+  )
 })