diff --git a/metaflow/parameters.py b/metaflow/parameters.py
index 48c561b77d6..eb5de9507c9 100644
--- a/metaflow/parameters.py
+++ b/metaflow/parameters.py
@@ -183,7 +183,10 @@ def _check_type(self, val, deploy_time):
 
         if isinstance(self.parameter_type, list):
             if not any(isinstance(val, x) for x in self.parameter_type):
-                msg += "Expected one of the following %s." % TYPES[self.parameter_type]
+                msg += "Expected one of the following %s." % ", ".join(
+                    TYPES.get(t, getattr(t, "__name__", str(t)))
+                    for t in self.parameter_type
+                )
                 raise ParameterFieldTypeMismatch(msg)
             return str(val) if self.return_str else val
         elif self.parameter_type in TYPES:
diff --git a/test/unit/test_deploy_time_field.py b/test/unit/test_deploy_time_field.py
new file mode 100644
index 00000000000..cbc841a646e
--- /dev/null
+++ b/test/unit/test_deploy_time_field.py
@@ -0,0 +1,80 @@
+"""Tests for DeployTimeField._check_type.
+
+@trigger / @trigger_on_finish wrap a callable attribute in a DeployTimeField
+whose parameter_type is a *list* of acceptable types, e.g. [str, dict] for
+`event=` and [list, dict, tuple] for `event={"parameters": ...}`. When the
+user's function returns something outside that set, the mismatch message has
+to be buildable -- otherwise the user never sees why their function was
+rejected.
+"""
+
+import pytest
+
+import metaflow.parameters as parameters
+from metaflow.exception import ParameterFieldTypeMismatch
+from metaflow.parameters import DeployTimeField, ParameterContext
+
+
+@pytest.fixture(autouse=True)
+def parameter_context(monkeypatch):
+    """DeployTimeField.__call__ curries the module-level context onto the
+    user function; the CLI normally installs it via set_parameter_context()."""
+    monkeypatch.setattr(
+        parameters,
+        "context_proto",
+        ParameterContext(
+            flow_name="TriggerFlow",
+            user_name="tester",
+            parameter_name=None,
+            logger=lambda *a, **kw: None,
+            ds_type="local",
+            configs=None,
+        ),
+    )
+
+
+def _field(parameter_type, value):
+    return DeployTimeField(
+        "event", parameter_type, None, lambda ctx, deploy_time: value, False
+    )
+
+
+@pytest.mark.parametrize(
+    "parameter_type, expected_names",
+    [
+        ([str, dict], ["str", "dict"]),
+        ([list, dict, tuple], ["list", "dict", "tuple"]),
+    ],
+    ids=["trigger_event", "trigger_event_parameters"],
+)
+def test_list_of_types_reports_the_mismatch(parameter_type, expected_names):
+    field = _field(parameter_type, 42)
+    with pytest.raises(ParameterFieldTypeMismatch) as excinfo:
+        field(deploy_time=True)
+    message = str(excinfo.value)
+    for name in expected_names:
+        assert name in message
+
+
+@pytest.mark.parametrize(
+    "parameter_type, value",
+    [
+        ([str, dict], "an-event-name"),
+        ([str, dict], {"name": "an-event-name"}),
+        ([list, dict, tuple], ["a", "b"]),
+        ([list, dict, tuple], ("a", "b")),
+    ],
+)
+def test_list_of_types_accepts_any_listed_type(parameter_type, value):
+    assert _field(parameter_type, value)(deploy_time=True) == value
+
+
+def test_single_type_mismatch_still_reports(parameter_type=int):
+    field = _field(parameter_type, "not-an-int")
+    with pytest.raises(ParameterFieldTypeMismatch) as excinfo:
+        field(deploy_time=True)
+    assert "Expected a int." in str(excinfo.value)
+
+
+def test_single_type_match_still_passes():
+    assert _field(int, 7)(deploy_time=True) == 7