Are there any NULLs?

On one of the projects we use Oracle 11g database with Label Security enabled. Last week I had a problem with adding NOT NULL constraint to a column. This query failed due to some NULL values allegedly being present:

ALTER TABLE tbl$portfolio MODIFY seeded DEFAULT 0 NOT NULL;

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That came to me as a surprise as I was pretty sure there were no NULL values - this query returned zero:

SELECT COUNT(*) FROM tbl$portfolio WHERE seeded IS NULL

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The problem was resolved by disabling Label Security policy temporarily:

	exec SA_POLICY_ADMIN.DISABLE_TABLE_POLICY(policy_name => 'SEC_POLICY', schema_name => 'APP_OWNER', table_name => 'TBL$PORTFOLIO');
	alter table TBL$PORTFOLIO modify SEEDED default 0 not null;
	exec SA_POLICY_ADMIN.ENABLE_TABLE_POLICY(policy_name => 'SEC_POLICY', schema_name => 'APP_OWNER', table_name => 'TBL$PORTFOLIO');

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I'm not familiar enough with Oracle Label Security (OLS) internal workings to explain what was the nature of the problem, but at least I could work around it and move on. If I find out something more then I'll post an update here.