diff --git a/zipline/assets/asset_writer.py b/zipline/assets/asset_writer.py
index b8190cdb..06e19a6e 100644
--- a/zipline/assets/asset_writer.py
+++ b/zipline/assets/asset_writer.py
@@ -168,7 +168,6 @@ class AssetDBWriter(with_metaclass(ABCMeta)):
             sa.Column(
                 'sid',
                 sa.Integer,
-                nullable=False,  # in case constraints is False
                 primary_key=constraints,
             ),
             sa.Column('symbol', sa.Text),
@@ -185,7 +184,6 @@ class AssetDBWriter(with_metaclass(ABCMeta)):
             sa.Column(
                 'exchange_id',
                 sa.Integer,
-                nullable=False,  # in case constraints is False
                 primary_key=constraints,
             ),
             sa.Column('exchange', sa.Text),
@@ -197,7 +195,6 @@ class AssetDBWriter(with_metaclass(ABCMeta)):
             sa.Column(
                 'root_symbol_id',
                 sa.Integer,
-                nullable=False,  # in case constraints is False
                 primary_key=constraints,
             ),
             sa.Column('root_symbol', sa.Text),
@@ -216,7 +213,6 @@ class AssetDBWriter(with_metaclass(ABCMeta)):
             sa.Column(
                 'sid',
                 sa.Integer,
-                nullable=False,  # in case constraints is False
                 primary_key=constraints,
             ),
             sa.Column('symbol', sa.Text),