BCS: Association Error

You are developing an association between BCS entities. You encouner an error like:
Exception handed to HandleRuntimeException.HandleException System.InvalidOperationException: The Association with Name 'SomethingAssociationNavigator' expects '1' EntityInstances (External Items) as input, but '0' were supplied.
Maybe you have defined the finder method of the entity but you missed the specific finder method or you defined a specific finder method but it has a faulty parameter definition.