Print preview for Schema Compare gave error "Unable to get root pair of schemas" (4279738)

When doing a Schema Compare and then a File | Print Preview, I get the following error:

System.ApplicationException
Unable to get root pair of schemas
Stack Trace:
at Quest.Compare.General.UI.CNSPrintReport.FillReport(CNSPrintEventArgs args)
at Quest.Compare.General.UI.SchemaCompareBase.SchemaCompareBasePanel.Print()
at Quest.Compare.Schema.DB2.Schema.Wrapper.DB2SchemaCompareForm.DoPrint(Boolean prompt)
at Quest.Toad.Gui.MainForm.printButton_ItemClick(Object sender, ItemClickEventArgs e)
at DevExpress.XtraBars.BarItem.OnClick(BarItemLink link)
at DevExpress.XtraBars.BarBaseButtonItem.OnClick(BarItemLink link)
at DevExpress.XtraBars.BarItemLink.OnLinkClick()
at DevExpress.XtraBars.BarItemLink.OnLinkAction(BarLinkAction action, Object actionArgs)
at DevExpress.XtraBars.BarButtonItemLink.OnLinkAction(BarLinkAction action, Object actionArgs)
at DevExpress.XtraBars.BarItemLink.OnLinkActionCore(BarLinkAction action, Object actionArgs)
at DevExpress.XtraBars.ViewInfo.BarSelectionInfo.ClickLink(BarItemLink link)
at DevExpress.XtraBars.ViewInfo.BarSelectionInfo.UnPressLink(BarItemLink link)
at DevExpress.XtraBars.Controls.CustomLinksControl.OnMouseUp(MouseEventArgs e)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)