diff --git a/layercake/layercake/dynamodb.py b/layercake/layercake/dynamodb.py
index 319c77a..4fb2dd9 100644
--- a/layercake/layercake/dynamodb.py
+++ b/layercake/layercake/dynamodb.py
@@ -23,6 +23,8 @@ DELIMITER = os.getenv('DYNAMODB_DELIMITER', '#')
 
 
 logger = Logger(__name__)
+serializer = TypeSerializer()
+deserializer = TypeDeserializer()
 
 
 def _serialize_python_type(value: Any) -> str | dict | list:
@@ -42,14 +44,12 @@ def _serialize_python_type(value: Any) -> str | dict | list:
 
 
 def serialize(value: dict) -> dict:
-    serializer = TypeSerializer()
     return {
         k: serializer.serialize(_serialize_python_type(v)) for k, v in value.items()
     }
 
 
 def deserialize(value: dict) -> dict:
-    deserializer = TypeDeserializer()
     return {k: deserializer.deserialize(v) for k, v in value.items()}
 
 
@@ -128,7 +128,7 @@ class PartitionKey(Key):
     """Represents a partition key for DynamoDB queries"""
 
     def __init__(self, pk: str) -> None:
-        super().__init__(**{PK: pk, SK: None})
+        super().__init__(**{PK: pk})
 
     def expr_attr_name(self) -> dict:
         return {'#pk': PK}
@@ -670,7 +670,7 @@ class DynamoDBCollection:
         last_key = _startkey_b64encode(res['last_key']) if res['last_key'] else None
 
         # Remove prefix from Sort Key if `key[SK]` is a PrefixKey
-        if isinstance(key[SK], PrefixKey):
+        if isinstance(key.get(SK), PrefixKey):
             prefix = key[SK].prefix
             items = [item | {SK: item[SK].removeprefix(prefix)} for item in items]