Copy attribute from Project Area to Other Project Area programmatically

We have 2 Project Areas(A,B) sharing Process from different Project Area(C).
We have 2 Workitem types Defect,Story

Usecase 1: With in Same Project Area (A)
I am trying to create New Defect Workitem by copying over some of the fields from existing Defect Workitem.
Result:
Works completely fine for Built in and Custom Attributes

Usecase 2:Betweeen Project Areas(A and B)
I am trying to create New Defect Workitem present in PA "B" by copying over some of the fields from existing Defect Workitem present in PA "A".
Result:
Getting below error,

Exception in thread "AWT-EventQueue-0" org.eclipse.core.runtime.AssertionFailedException: assertion failed:
    at org.eclipse.core.runtime.Assert.isTrue(Assert.java:110)
    at org.eclipse.core.runtime.Assert.isTrue(Assert.java:96)
    at com.ibm.team.workitem.common.internal.model.impl.WorkItemImpl.setValue(WorkItemImpl.java:2909)
    at com.bosch.masterchild.almchildproblem.WorkItemInitialisation.execute(WorkItemInitialisation.java:48)

Why this is not Possible between Project Areas though they are having Same Process.

Code:
Getting workitemtype from ProjectArea A
workItemType = this.workItemService.findWorkItemType(this.projArea, WIConstants.typeIdentifier, null);
Getting identifiers from Workitems in ProjectArea A
WorkItemInit operation =
          new WorkItemInit(this.defectObj.getSummaryWI(), this.category,
              this.defectObj.getDescriptionWI(), this.defectObj.getSeverity(), this.engIdent, this.engIAtt,
              this.dpIdent, this.dpIAttr);
      IWorkItemHandle handle = operation.run(workItemType, null);

public class WorkItemInit extends WorkItemOperation {

  private final String fSummary;
  Identifier<ISeverity> severty;
  private final String fDescription;
  private final ICategoryHandle fCategory;
  private final Identifier engValue;
  private final IAttribute attrEng;
  private final Identifier dpValue;
  private final IAttribute dpAttr;

  public WorkItemInit(final String summary, final ICategoryHandle category, final String description,
      final Identifier<ISeverity> severity, final Identifier engValue, final IAttribute attrEng,
      final Identifier dpValue, final IAttribute dpAttr) {
    super("Initializing Work Item");
    this.fSummary = summary;
    this.fCategory = category;
    this.fDescription = description;
    this.severty = severity;
   
    // Custom Attribute
    this.engValue = engValue;
    this.attrEng = attrEng;
    this.dpValue = dpValue;
    this.dpAttr = dpAttr;
  }

@Override
  protected void execute(final WorkItemWorkingCopy workingCopy, final IProgressMonitor monitor)
      throws TeamRepositoryException {
    IWorkItem workItem = workingCopy.getWorkItem();
    workItem.setHTMLSummary(XMLString.createFromPlainText(this.fSummary));
    workItem.setCategory(this.fCategory);
// Setting the Identifier and value fetched above to same workitem type in other Project Area and has enumerations same.
    workItem.setValue(this.attrEng, this.engValue); // Custom Attribute
    workItem.setHTMLDescription(XMLString.createFromPlainText(this.fDescription));
    workItem.setSeverity(this.severty);
  }