Does anyone know why the following query line:

List<Contact> contacts = new List<Contact> ([Select Id, Email from Contact where IsPersonAccount = false]);

Yields this error message:

Error: Invalid Data.
Review all error messages below to correct your data.
Apex trigger LeadAssignmentFromImagitas caused an unexpected exception, contact your administrator: LeadAssignmentFromImagitas: execution of BeforeInsert caused by: System.QueryException: Non-selective query against large object type (more than 100000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null) 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times): Trigger.LeadAssignmentFromImagitas: line 18, column 9

?

The query returns only 58 rows.

Any help is greatly appreciated.  Thank you.