How to add unique key constraint on CRM entity column

Hello,

I am trying to create an unique key constraint on the CRM entity via database.

When I execute the alter script it throws an error as,

Msg 1505, Level 16, State 1, Line 1

The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name 'dbo.LeadBase' and the index name 'uc_lead'. The duplicate key value is (<NULL>, <NULL>).

Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.
The statement has been terminated.

There are some records in the database which have null values.

How can we add the constraint from database?