showModal:fail parameter error: parameter.content should be String...

小程序showModal问题

let content = '你已在房间' + model.roomId + '，直接进入房间，还是退出当前所在房间？'
          wx.showModal({
            title: '提示',
            content: content,
            showCancel: true,
            confirmText: '进入',
            cancelText: '退出',
            success: res => {
                if (res.confirm) {
                  wx.navigateTo({
                    url: '../gameroom/gameroom?' +
                      '&roomId=' + model.roomId
                  })
                }else if (res.cancel) {
                  callback()
                }
            },
          })

以上代码就会报我标题的错，只要把单引号改成双引号就行了

let content = "你已在房间" + model.roomId + "，直接进入房间，还是退出当前所在房间？"