diff --git a/tests/testthat/test-as.R b/tests/testthat/test-as.R
new file mode 100644
index 0000000..64eb274
--- /dev/null
+++ b/tests/testthat/test-as.R
@@ -0,0 +1,28 @@
+test_that("as_api_object() fails informatively with bad x", {
+  simple_class <- S7::new_class(
+    "range",
+    properties = list(
+      start = S7::class_numeric,
+      end = S7::class_numeric
+    )
+  )
+  expect_no_error(as_api_object(NULL, simple_class))
+  expect_error(
+    as_api_object(1, simple_class),
+    class = "rapid_error_unknown_coercion"
+  )
+})
+
+test_that("as_api_object() warns about unexpected fields", {
+  simple_class <- S7::new_class(
+    "range",
+    properties = list(
+      start = S7::class_numeric,
+      end = S7::class_numeric
+    )
+  )
+  expect_warning(
+    {as_api_object(list(start = 1, end = 2, a = 1), simple_class)},
+    class = "rapid_warning_extra_names"
+  )
+})