Passing Key Value from Task to BO->Method

Hi,

I created BO for fetching the Material data basing on Material Provided

Now, in workflow I have created a task container.

I am getting the below error.

Value of expression '&_WI_OBJECT_ID.MATERIAL&' cannot be used as the target of an assignment

Please suggest how to pass Key Values to Business Object Method(Method has importing Parameter as MATNR.)

Thank.

Regards,

Rafi