diff --git a/fin_depo/investbank_nordnet.py b/fin_depo/investbank_nordnet.py
index 8444835..7123606 100644
--- a/fin_depo/investbank_nordnet.py
+++ b/fin_depo/investbank_nordnet.py
@@ -14,6 +14,7 @@ import logging
 from decimal import Decimal
 
 import fin_defs
+from frozendict import frozendict
 
 from .data import Depo, DepoGroup, DepoSingle
 
@@ -38,6 +39,7 @@ def asset_from_instrument_json(json) -> fin_defs.Asset | None:
     exchange_id = json['tradables'][0]['mic']
     return fin_defs.Stock(symbol, fin_defs.EXCHANGES_BY_IDS[exchange_id])
 
+EMPTY_DICT: dict[str,str | int] = frozendict()
 
 class NordnetDepoFetcher:
     """Depository fetcher for [Nordnet](https://nordnet.dk), the online investment bank.
@@ -56,16 +58,22 @@ class NordnetDepoFetcher:
     """
 
     def __init__(self, session, username: str, password: str):
-        assert session is not None, 'missing session'
         self.session = session
         self.username = username
         self.password = password
         self.is_logged_in = False
 
-        assert self.username is not None, 'Username is missing'
-        assert self.password is not None, 'Password is missing'
+        if self.session is None:
+            msg = 'Session argument is missing'
+            raise TypeError(msg)
+        if self.username is not None:
+            msg = 'Username argument s missing'
+            raise TypeError(msg)
+        if self.password is not None:
+            msg = 'Password argument is missing'
+            raise TypeError(msg)
 
-    def get_json(self, url: str, params: dict[str, str | int] = {}) -> dict:
+    def get_json(self, url: str, params: dict[str, str | int] = EMPTY_DICT)-> dict:
         if not url.startswith(API_ROOT):
             msg = f'Given url must be located below API ROOT: {url}'
             raise ValueError(msg)
@@ -109,7 +117,7 @@ class NordnetDepoFetcher:
 
         json_accounts = self.get_json(API_ACCOUNTS)
 
-        nested = []
+        nested: list[Depo] = []
         for json_account in json_accounts:
             account_id = json_account['accid']
 
@@ -148,7 +156,7 @@ class NordnetDepoFetcher:
             )
 
         return DepoGroup(
-            name=f'Nordnet',
+            name='Nordnet',
             updated_time=now,
             nested=nested,
         )