diff --git a/api/kiwi_vpn_api/db/models.py b/api/kiwi_vpn_api/db/models.py
index 421dbed..633e78a 100644
--- a/api/kiwi_vpn_api/db/models.py
+++ b/api/kiwi_vpn_api/db/models.py
@@ -18,7 +18,7 @@ class User(ORMBaseModel):
     __tablename__ = "users"
 
     name = Column(String, primary_key=True, index=True)
-    password = Column(String)
+    password = Column(String, nullable=False)
 
     capabilities: list[UserCapability] = relationship(
         "UserCapability", lazy="joined", cascade="all, delete-orphan"
@@ -60,14 +60,14 @@ class DistinguishedName(ORMBaseModel):
     id = Column(Integer, primary_key=True, autoincrement=True)
 
     owner_name = Column(String, ForeignKey("users.name"))
-    cn_only = Column(Boolean, default=True)
+    cn_only = Column(Boolean, default=True, nullable=False)
     country = Column(String(2))
     state = Column(String)
     city = Column(String)
     organization = Column(String)
     organizational_unit = Column(String)
     email = Column(String)
-    common_name = Column(String)
+    common_name = Column(String, nullable=False)
 
     UniqueConstraint(
         country,
@@ -86,7 +86,13 @@ class Certificate(ORMBaseModel):
     id = Column(Integer, primary_key=True, autoincrement=True)
 
     owner_name = Column(String, ForeignKey("users.name"))
-    dn_id = Column(Integer, ForeignKey("distinguished_names.id"))
+    dn_id = Column(
+        Integer,
+        ForeignKey("distinguished_names.id"),
+        nullable=False,
+    )
     expiry = Column(DateTime, default=datetime.datetime.now)
 
-    distinguished_name = relationship("DistinguishedName", lazy="joined")
+    distinguished_name: DistinguishedName = relationship(
+        "DistinguishedName", lazy="joined"
+    )