Odd problem with DB_DataObject

Hi, Hopefully this is a really stupid mistake on my part, but I'm stumped... I've narrowed my problem down to this simple test case: class TestClass extends DB_DataObject { function TestClass() { $this->testvar = 'hello world'; } function testMethod() { } } $TESTOBJ = new TestClass(); var_dump($TESTOBJ); $TESTOBJ->testMethod(); var_dump($TESTOBJ); When var_dump() is called the first time, the object's contents are displayed as you'd expect, with $testvar included. However, after calling testMethod(), the second var_dump() simply displays 'NULL'. I can't work out what's happening there to destroy the object. If I remove 'extends DB_DataObject' this test case works fine (outputting the object with $testvar included both times), so it's obviously something in DB_DataObject that I'm not fully grasping... Can anyone put me out of my misery? Many thanks... Phil