Extension of protocol with method which adds default parameter

I'm used to having default parameters inside protocols using extensions, as protocol declarations themselves can't have them, like this:

protocol Controller {
   func fetch(forPredicate predicate: NSPredicate?)
}

extension Controller {
   func fetch(forPredicate predicate: NSPredicate? = nil) {
      return fetch(forPredicate: nil)
   }
}

Worked perfectly for me.

Now I have the next situation, I have one specific protocol for specific kind of controller:

protocol SomeSpecificDatabaseControllerProtocol {
    //...
    func count(forPredicate predicate: NSPredicate?) -> Int
}

And protocol-extension with implementations of default methods for controllers:

protocol DatabaseControllerProtocol {
    associatedtype Entity: NSManagedObject
    func defaultFetchRequest() -> NSFetchRequest<Entity>
    var context: NSManagedObjectContext { get }
}

extension DatabaseControllerProtocol {
    func save() {
        ...
    }

    func get() -> [Entity] {
        ...
    }

    func count(forPredicate predicate: NSPredicate?) -> Int {
        ...
    }

    //.....
}

My issue is when I'm trying to add method with default parameter to the SomeSpecificDatabaseControllerProtocol extension, I'm receiving a compile-time error, that concrete class conforming to SomeSpecificDatabaseControllerProtocol doesn't conform to the protocol (happens only if I extend protocol):

class SomeClassDatabaseController: SomeSpecificDatabaseControllerProtocol, DatabaseControllerProtocol {...}

What am I missing?