diff --git a/backend/app/models/session.py b/backend/app/models/session.py
index 274e13b..6def90e 100644
--- a/backend/app/models/session.py
+++ b/backend/app/models/session.py
@@ -29,8 +29,13 @@ class RefreshToken(Base):
     # RS256 JWTs are ~420+ chars; 512 is sufficient and matches the migration.
     token = Column(String(512), unique=True, index=True, nullable=False)
     expires_at = Column(DateTime(timezone=True), nullable=False)
-    is_active = Column(Boolean, default=True)
-    created_at = Column(DateTime(timezone=True), server_default=func.now())
-    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+    is_active = Column(Boolean, nullable=False, default=True, server_default="true")
+    created_at = Column(DateTime(timezone=True), nullable=False, server_default=func.now())
+    updated_at = Column(
+        DateTime(timezone=True),
+        nullable=False,
+        server_default=func.now(),
+        onupdate=func.now(),
+    )
 
     user = relationship("User", back_populates="refresh_tokens")