diff --git a/pynamodb/attributes.py b/pynamodb/attributes.py
index 957e148f..80034f76 100644
--- a/pynamodb/attributes.py
+++ b/pynamodb/attributes.py
@@ -6,12 +6,14 @@
 import collections.abc
 import json
 import time
+import typing
 import warnings
 from base64 import b64encode, b64decode
 from copy import deepcopy
 from datetime import datetime
 from datetime import timedelta
 from datetime import timezone
+from decimal import Decimal
 from inspect import getfullargspec
 from inspect import getmembers
 from typing import Any, Callable, Dict, Generic, List, Mapping, Optional, TypeVar, Type, Union, Set, overload, Iterable
@@ -178,7 +180,12 @@ def serialize(self, value: Any) -> Any:
         see `DynamoDB.Client.get_item API reference
         <https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/dynamodb.html#DynamoDB.Client.get_item>`_.
         """
-        return value
+        if value is None or isinstance(value, str):
+            return value
+        raise TypeError(
+            f"UnicodeAttribute expected str for '{self.attr_name}', "
+            f"got {type(value).__name__}"
+        )
 
     def deserialize(self, value: Any) -> Any:
         """
@@ -741,6 +748,13 @@ def serialize(self, value):
         """
         Encode numbers as JSON
         """
+        if isinstance(value, bool):
+            raise TypeError("Boolean values are not allowed for NumberAttribute")
+
+        if not isinstance(value, (int, float, Decimal)):
+            raise TypeError(
+                f"Expected int, float, or Decimal for NumberAttribute, got {type(value).__name__}"
+            )
         return json.dumps(value)
 
     def deserialize(self, value):
diff --git a/tests/test_attributes.py b/tests/test_attributes.py
index 862a492c..0d4a69a0 100644
--- a/tests/test_attributes.py
+++ b/tests/test_attributes.py
@@ -342,6 +342,24 @@ def test_number_deserialize(self):
         assert attr.deserialize('3.141') == 3.141
         assert attr.deserialize('12345678909876543211234234324234') == 12345678909876543211234234324234
 
+    @pytest.mark.parametrize(
+        "value",
+        [
+            "42",  # string
+            True,  # bool
+            None,  # None
+            [1, 2, 3],  # list
+            {"a": 1},  # dict
+            {1, 2, 3},  # set
+            object(),  # arbitrary object
+        ],
+    )
+    def test_serialize_invalid_types(self, value):
+        attr = NumberAttribute()
+        with pytest.raises(TypeError) as exc_info:
+            attr.serialize(value)
+            assert "Expected int, float, or Decimal" in str(exc_info.value)
+
     def test_number_set_deserialize(self):
         """
         NumberSetAttribute.deserialize
@@ -399,6 +417,12 @@ def test_unicode_deserialize(self):
         assert attr.deserialize('') == ''
         assert attr.deserialize(None) is None
 
+    @pytest.mark.parametrize("value", [123, 1.23, True, False, [], {}, object()])
+    def test_unicode_serialize_invalid(self, value):
+        attr = UnicodeAttribute()
+        with pytest.raises(TypeError):
+            attr.serialize(value)
+
     def test_unicode_set_serialize(self):
         """
         UnicodeSetAttribute.serialize
diff --git a/tests/test_model.py b/tests/test_model.py
index da54303b..11b21751 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -727,12 +727,12 @@ def test_delete_doesnt_do_validation_on_null_attributes(self):
         """
         with patch(PATCH_METHOD) as req:
             req.return_value = {}
-            CarModel('foo').delete()
+            CarModel(1234).delete()
 
         with patch(PATCH_METHOD) as req:
             req.return_value = {}
             with CarModel.batch_write() as batch:
-                car = CarModel('foo')
+                car = CarModel(1234)
                 batch.delete(car)
 
     @patch('time.time')
@@ -1046,7 +1046,7 @@ def fake_dynamodb(*args, **kwargs):
 
     def test_count_no_hash_key(self):
         with pytest.raises(ValueError):
-            UserModel.count(filter_condition=(UserModel.zip_code <= '94117'))
+            UserModel.count(filter_condition=(UserModel.zip_code <= 94117))
 
     def test_index_count(self):
         """