I am trying to delete an Action Plan from an account record.  Here is the error that I get:

There were custom validation error(s) encountered while saving the affected record(s). The first validation error encountered was "Apex trigger ActionPlanTrigger caused an unexpected exception, contact your administrator: ActionPlanTrigger: execution of BeforeDelete 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.ActionPlanTrigger: line 18, column 1". 

Any help on this would be appreciated!  Thanks, in advance!

Steve