I did a quick search in the mailing list, but couldn't find a related issue. The following rule worked fine in 5.3.0.CR1 (and earlier) ... when $party : ImmutableParty(addresses == null || addresses.size() == 0) then ... ImmutableParty has a getter for addresses and addresses is a collection. When upgrading to 5.3.0.Final, I get this error: Caused by: java.lang.RuntimeException: Predicate 'addresses == null || addresses.size() == 0' must be a Boolean expression [Line: 17, Column: 32] : [Rule name='party - address - at least one'] This is with mvel 2.1 beta 6. -- View this message in context: http://drools.46999.n3.nabble.com/drools-5-3-0-Final-Predicate-must-be-a-... Sent from the Drools: User forum mailing list archive at Nabble.com.