Single character shortcuts are invalid

//代码占位符
Rectangle{
     signal retrunSignal()
     Shortcut { 
         sequence: "Z"; 
         context: Qt.ApplicationShortcut ;
         autoRepeat: true; 
         onActivated: returnButton.clicked(); 
         enabled: returnButton.visible ; 
     }
   TextField {
        focus: true
         validator: RegExpValidator{ regExp: /^[0-9]+$/ ;}
   }
     Button{
        id:returnButton
          onClicked: {
                retrunSignal()
            }
     }

}