diff --git a/accountant/models/operations.py b/accountant/models/operations.py
index 07326fc..e85ae92 100644
--- a/accountant/models/operations.py
+++ b/accountant/models/operations.py
@@ -149,7 +149,7 @@ class Operation(db.Model):
     @classmethod
     def get_categories_for_range(cls, account, begin, end):
         """Get category list for a specific time period."""
-        if isinstance(account, int) or isinstance(account, str):
+        if isinstance(account, (int, str)):
             account_id = account
         else:
             account_id = account.id
@@ -182,7 +182,7 @@ class Operation(db.Model):
     @classmethod
     def get_ohlc_per_day_for_range(cls, account, begin=None, end=None):
         """Get Opening, High, Low, Closing per day for a specific range"""
-        if isinstance(account, int) or isinstance(account, str):
+        if isinstance(account, (int, str)):
             account_id = account
         else:
             account_id = account.id