diff --git a/mautrix_telegram/db/portal.py b/mautrix_telegram/db/portal.py
index dcc418d8..bae8c76a 100644
--- a/mautrix_telegram/db/portal.py
+++ b/mautrix_telegram/db/portal.py
@@ -15,7 +15,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 from typing import Optional
 
-from sqlalchemy import Column, Integer, String, Boolean, Text
+from sqlalchemy import Column, Integer, String, Boolean, Text, func
 
 from mautrix.types import RoomID
 from mautrix.util.db import Base
@@ -53,4 +53,4 @@ class Portal(Base):
 
     @classmethod
     def get_by_username(cls, username: str) -> Optional['Portal']:
-        return cls._select_one_or_none(cls.c.username == username)
+        return cls._select_one_or_none(func.lower(cls.c.username) == username)
diff --git a/mautrix_telegram/db/puppet.py b/mautrix_telegram/db/puppet.py
index c647b8d3..46143b98 100644
--- a/mautrix_telegram/db/puppet.py
+++ b/mautrix_telegram/db/puppet.py
@@ -16,7 +16,7 @@
 from typing import Optional, Iterable
 
 from sqlalchemy import Column, Integer, String, Boolean
-from sqlalchemy.sql import expression
+from sqlalchemy.sql import expression, func
 
 from mautrix.types import UserID, SyncToken
 from mautrix.util.db import Base
@@ -53,7 +53,7 @@ class Puppet(Base):
 
     @classmethod
     def get_by_username(cls, username: str) -> Optional['Puppet']:
-        return cls._select_one_or_none(cls.c.username == username)
+        return cls._select_one_or_none(func.lowercase(cls.c.username) == username)
 
     @classmethod
     def get_by_displayname(cls, displayname: str) -> Optional['Puppet']:
diff --git a/mautrix_telegram/db/user.py b/mautrix_telegram/db/user.py
index 36b9d7b7..81affda2 100644
--- a/mautrix_telegram/db/user.py
+++ b/mautrix_telegram/db/user.py
@@ -15,7 +15,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 from typing import Optional, Iterable, Tuple
 
-from sqlalchemy import Column, ForeignKey, ForeignKeyConstraint, Integer, String
+from sqlalchemy import Column, ForeignKey, ForeignKeyConstraint, Integer, String, func
 
 from mautrix.types import UserID
 from mautrix.util.db import Base
@@ -46,7 +46,7 @@ class User(Base):
 
     @classmethod
     def get_by_username(cls, username: str) -> Optional['User']:
-        return cls._select_one_or_none(cls.c.tg_username == username)
+        return cls._select_one_or_none(func.lower(cls.c.tg_username) == username)
 
     @property
     def contacts(self) -> Iterable[TelegramID]:
diff --git a/mautrix_telegram/portal/base.py b/mautrix_telegram/portal/base.py
index bbea89a1..6dc32b32 100644
--- a/mautrix_telegram/portal/base.py
+++ b/mautrix_telegram/portal/base.py
@@ -354,8 +354,10 @@ class BasePortal(ABC):
         if not username:
             return None
 
+        username = username.lower()
+
         for _, portal in cls.by_tgid.items():
-            if portal.username and portal.username.lower() == username.lower():
+            if portal.username and portal.username.lower() == username:
                 return portal
 
         dbportal = DBPortal.get_by_username(username)
diff --git a/mautrix_telegram/puppet.py b/mautrix_telegram/puppet.py
index b8f7c24f..cf9c478f 100644
--- a/mautrix_telegram/puppet.py
+++ b/mautrix_telegram/puppet.py
@@ -384,8 +384,10 @@ class Puppet(CustomPuppetMixin):
         if not username:
             return None
 
+        username = username.lower()
+
         for _, puppet in cls.cache.items():
-            if puppet.username and puppet.username.lower() == username.lower():
+            if puppet.username and puppet.username.lower() == username:
                 return puppet
 
         dbpuppet = DBPuppet.get_by_username(username)
diff --git a/mautrix_telegram/user.py b/mautrix_telegram/user.py
index ea7e2032..3afce211 100644
--- a/mautrix_telegram/user.py
+++ b/mautrix_telegram/user.py
@@ -424,8 +424,10 @@ class User(AbstractUser, BaseUser):
         if not username:
             return None
 
+        username = username.lower()
+
         for _, user in cls.by_tgid.items():
-            if user.username and user.username.lower() == username.lower():
+            if user.username and user.username.lower() == username:
                 return user
 
         puppet = DBUser.get_by_username(username)