diff --git a/crudkit/core/base.py b/crudkit/core/base.py
index 0540ec0..dae5615 100644
--- a/crudkit/core/base.py
+++ b/crudkit/core/base.py
@@ -6,8 +6,8 @@ Base = declarative_base()
 @declarative_mixin
 class CRUDMixin:
     id = Column(Integer, primary_key=True)
-    created_at = Column(DateTime, default=func.now())
-    updated_at = Column(DateTime, default=func.now(), onupdate=func.now())
+    created_at = Column(DateTime, default=func.now(), nullable=False)
+    updated_at = Column(DateTime, default=func.now(), nullable=False, onupdate=func.now())
 
     def as_dict(self):
         # Combine all columns from all inherited tables