Issue with updateStructure webservice

Hello

We are trying to use updateStructure operation from Item structure version 2 web service. We wanted to update the quantity of an already existing component in a structure. However, we are seeing the following response with exception. Can anyone help to point to the actual cause of this issue. We are not pasisng UOM code at all, but still we are receiving this message. We are on SCM cloud R12

<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">

   <env:Header/>

   <env:Body>

      <env:Fault>

         <faultcode>env:Server</faultcode>

         <faultstring>JBO-27008: Attribute PrimaryUOMValue in view object PendingStructureHeaderEO_PendingComponent_StructureVOToComponentVO_ComponentVO cannot be set.</faultstring>