fix: resolve unreachable exception when schema is not a class

tests/test_lib.py

@@ -214,3 +214,13 @@ class TestEnum(Enum):
             (2, 2),
         ],
     } == convert(vol.Schema(vol.Coerce(TestEnum)))
+
+
+def test_invalid_schema():
+    # check if an exception is raised when an invalid schema is passed
+    try:
+        convert(vol.Schema(None))
+    except Exception as e:
+        assert str(e) == "Unable to convert schema: None"
+    else:
+        assert False, "Expected an exception to be raised"

voluptuous_serialize/__init__.py

@@ -118,7 +118,7 @@ def convert(schema, *, custom_serializer=None):
     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],