ITAB that hold objects: How to use the statement?

Hi all,

I created an internal table that hold instances of a class:

data: gt_errortable type standard table of ref to ZCL_ERROR with HEADER LINE.

Lets assume the class has only two attributes:

1. attribute_A
2. attribute_B

How to I have to do a read?

  READ TABLE gt_errortable  WITH KEY

     attribute_A = 'X'

     gives an error:

     No component with name attribute_A existing.

     Thanks

     Regards

     Mario