trading

While creating sales order iam getting the following errror for trading materials

Internal number assignment not defined (please enter number)

Message no. 06243

Diagnosis

The system cannot assign a number because internal number assignment has not been defined in this case.

Procedure

Enter a number that lies within the valid number range for the document.

if i enter normal material it is not giving any message

but if i remove order quantity of trading material in sales order it is not giving any message

am using item category group for trading banc and material type is zawa

wat might be the problem

plz can any suggesst me