InvocationTargetException in setter

[asookazian]

I have a setter in my SFSB:

@Name("merchantBillingGroupEditorUI")
@Scope(ScopeType.CONVERSATION)
@MeasureCalls
@Stateful
public class MerchantBillingGroupEditorUIBean implements
        MerchantBillingGroupEditorUI {
        public void setPercent(BigDecimal percent) {
            BigDecimal currentPercent = editor.getBillingGroup().getDiscountRate();  //the exception is triggered by this line!
            if (currentPercent.compareTo(percent) != 0)
              resetBaseQualEffectiveDate();
           editor.getBillingGroup().setDiscountRate(percent);
        }
    }

Exception method invoked in debugger:

public InvocationTargetException(Throwable target) {
     super((Throwable)null);  // Disallow initCause
        this.target = target;
    }

What is causing this and why is it happening (how to fix)?