Base Unit Of Measure error

Hi,

We get an error stating base unit of measure must have a value in item master when posting an created sales order. There is no Base Unit of measure set for that Item. But some how they have posted purchased receipt, Now it has 4 qty in the inventory. When we tried to post a negative journal for that item, Even then we get this error again. Is there any way to set the Base unit of measure for that Item. Any help would be appreciated.

Thanks,
Mathan