Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
.equalsIgnoreCase( mask ) )
		{
			throw new PropertyValueException( element, prop, value,
					PropertyValueException.DESIGN_EXCEPTION_VALUE_LOCKED );
		}
		
		
		if ( element.isVirtualElement( ) && element instanceof Cell )
		{
			String propName = prop.getName( );
			if ( Cell.COL_SPAN_PROP.equalsIgnoreCase( propName )
					|| Cell.ROW_SPAN_PROP.equalsIgnoreCase( propName )
					|| Cell.DROP_PROP.equalsIgnoreCase( propName )
					|| Cell.COLUMN_PROP.equalsIgnoreCase( propName ) )
			{
				throw new PropertyValueException(
						element,
						prop,
						value,
						PropertyValueException.DESIGN_EXCEPTION_PROPERTY_CHANGE_FORBIDDEN );
			}
		}
		if ( element instanceof MasterPage )
		{
			
			
			
			String propName = prop.getName( );
			if ( !( (MasterPage) element ).isCustomType( module )
					&& ( MasterPage.WIDTH_PROP.equals( propName ) || MasterPage.HEIGHT_PROP