Having a default value in a decimal field in base class forces Doctrine to try to save an instance even if not required.

I have 2 classes. Order and Branch. An Order class can have one or no branch assigned to it. My base class definition for a branch is simple. It just has a name.

The Order class has:
$this->hasColumn('branchId', 'integer');

and

$this->hasOne(
'Fds_Model_Branch as branch',
array(
'local' => 'branchId',
'foreign' => 'id',
)
);

The branch class had this:

$this->hasColumn('name', 'string', 50, array(
'notnull' => true,
'notblank' => true,
));

No hasMany relation back to Orders.

Using this setup, all my tests work.

I added this to the Branch class.
$this->hasColumn(
'localTax', 'decimal', null,
array(
'notnull' => true,
'default' => 0,
)
);

Now my tests fail with a validator error from the branch class for the name property whenever I try to save an order that has no branch associated to it. Pretty weird right? I mean, my order has no branch, and yet Doctrine tries to save one.

But if I change the definition of localTax to this:
$this->hasColumn('localTax', 'decimal');

Then my tests work as expected again. Problem is, I need to assign the default value of 0 to localTax instead of null.

Is this a bug?