diff --git a/favro_sync/favro_client.py b/favro_sync/favro_client.py
index dddbb43..3446d24 100644
--- a/favro_sync/favro_client.py
+++ b/favro_sync/favro_client.py
@@ -76,9 +76,16 @@ class FavroClient:
         session: requests.Session | None = None,
         read_only=True,
     ):
-        assert favro_org_id is not None
-        assert favro_username is not None
-        assert favro_password is not None
+        # Check arguments
+        if not isinstance(favro_org_id, str):
+            msg = f'favro_org_id must be str, but was {type(favro_org_id)}'
+            raise TypeError(msg)
+        if not isinstance(favro_username, str):
+            msg = f'favro_username must be str, but was {type(favro_username)}'
+            raise TypeError(msg)
+        if not isinstance(favro_password, str):
+            msg = f'favro_password must be str, but was {type(favro_password)}'
+            raise TypeError(msg)
 
         # Setup session
         self.session = session or requests.Session()
@@ -172,7 +179,7 @@ class FavroClient:
         response = self.session.get(URL_GET_TAG.format(tag_id=tag_id.raw_id))
         return TagInfo.from_json(response.json())
 
-    def get_custom_field(self, tag_id: CustomFieldId) -> CustomFieldInfo:
+    def get_custom_field(self, custom_field_id: CustomFieldId) -> CustomFieldInfo:
         response = self.session.get(
             URL_GET_CUSTOM_FIELD.format(custom_field_id=custom_field_id.raw_id),
         )