diff --git a/tests/test_lib.py b/tests/test_lib.py
index 6caf463..9d2317c 100644
--- a/tests/test_lib.py
+++ b/tests/test_lib.py
@@ -1,5 +1,6 @@
 from enum import Enum
 
+import pytest
 import voluptuous as vol
 
 from voluptuous_serialize import UNSUPPORTED, convert
@@ -214,3 +215,9 @@ class TestEnum(Enum):
             (2, 2),
         ],
     } == convert(vol.Schema(vol.Coerce(TestEnum)))
+
+
+@pytest.mark.parametrize("invalid_schema", [None, []])
+def test_invalid_schema(invalid_schema):
+    with pytest.raises(ValueError):
+        convert(vol.Schema(invalid_schema))
diff --git a/voluptuous_serialize/__init__.py b/voluptuous_serialize/__init__.py
index 135ffbf..40e5ed6 100644
--- a/voluptuous_serialize/__init__.py
+++ b/voluptuous_serialize/__init__.py
@@ -1,6 +1,6 @@
 """Module to convert voluptuous schemas to dictionaries."""
 
-from collections.abc import Mapping
+from collections.abc import Hashable, Mapping
 from enum import Enum
 
 import voluptuous as vol
@@ -112,13 +112,13 @@ def convert(schema, *, custom_serializer=None):
     if isinstance(schema, vol.Coerce):
         schema = schema.type
 
-    if schema in TYPES_MAP:
+    if isinstance(schema, Hashable) and schema in TYPES_MAP:
         return {"type": TYPES_MAP[schema]}
 
     if isinstance(schema, (str, int, float, bool)):
         return {"type": "constant", "value": schema}
 
-    if issubclass(schema, Enum):
+    if isinstance(schema, type) and issubclass(schema, Enum):
         return {
             "type": "select",
             "options": [(item.value, item.value) for item in schema],