[ruby-talk:5401] rb_gc_mark error message

Hi,

Anyone got ideas on what could cause the following error message;

/usr/local/lib/ruby/site_ruby/1.6/feldt/ai/ml/Predictions.rb:40: [BUG]
rb_gc_mark(): unknown data type 0x18(0x6107c128) non object

Predictions is a sub-class of NArray so it's TYPE should probably be
DATA. Line 40 reads:
@sub_predictions_hash[sub_predictions.predictor_id] = sub_predictions

but I'm unsure how rb_gc_mark is used. Do you know:

* What "actions" can cause rb_gc_mark to be called? 
* Is rb_gc_mark called as part of the gc itself (my guess is not) or
called when objects are marked for gc (my guess)? 
* If the latter then is the scenario likely that it's the object
that is "overwritten" by sub_predictions that is the faulty object?
* Any known ways that an object can get an invalid type?

Any info or hints related to this bug or rb_gc_mark in general greatly
appreciated!

Regards,

Robert