diff --git a/lib/cancan/model_adapters/active_record_adapter.rb b/lib/cancan/model_adapters/active_record_adapter.rb
index 33766c0a..d0a406be 100644
--- a/lib/cancan/model_adapters/active_record_adapter.rb
+++ b/lib/cancan/model_adapters/active_record_adapter.rb
@@ -198,11 +198,11 @@ def merge_non_empty_conditions(behavior, conditions_hash, sql)
         conditions = sanitize_sql(conditions_hash)
         case sql
         when true_sql
-          behavior ? true_sql : "not (#{conditions})"
+          behavior ? true_sql : "(not (#{conditions}) OR (#{conditions}) IS NULL)"
         when false_sql
           behavior ? conditions : false_sql
         else
-          behavior ? "(#{conditions}) OR (#{sql})" : "not (#{conditions}) AND (#{sql})"
+          behavior ? "(#{conditions}) OR (#{sql})" : "(not (#{conditions}) OR (#{conditions}) IS NULL) AND (#{sql})"
         end
       end