External calls from CS_CHARGE_DETAILS_CUHK.Call_Pricing_Item_Pre

Has anyone had success calling a secondary procedure from the CS_CHARGE_DETAILS_CUHK.Call_Pricing_Item_Pre procedure?

Every attempt I make causes a blank error message to be displayed.

CS_PRICING_ITEM_PKG.Call_Pricing_Item

   CS_CHARGE_DETAILS_CUHK.Call_Pricing_Item_Pre

       XX_QP_CALL.Call_QP

           x_list_price := 99;

    CS_CHARGE_DETAILS_CUHK.Call_Pricing_Item_Pre

        <crash>

Using FND Debug, I can get debug messages, in CS_CHARGE_DETAILS_CUHK.Call_Pricing_Item_Pre, to display the list price.

But the Service Request Charges form has a blank error.

I can't get any additional log messages in Fnd_Log_messages following the external procedure call.