What is the process for creating a partial index for an email field when email field is not present or as empty field in MongoDB?

I'm using MongoDB and want to create a partial index on the email field, but I only want it to apply to documents where the email field exists (i.e., email: { $exists: true }). How can I create this partial index? Here is what I have so far: db.collection.createIndex( { email: 1 }, { partialFilterExpression: { email: { $exists: true } } } ); When I use this index I don't add a new user without an email or an empty field Required: when I add an empty email or null or without email field then it should created many times but I received the error of duplication when I don't want to add an email with a partial Index Thanks in advance for your help!