使用c#给visual 's Shape赋值.文本导致异常

doc = app.Documents.Add(fileInfo[i].FullName);
Vis.Shapes shapes = doc.Pages[1].Shapes;
for (int j = 1; j <= shapes.Count; j++)
{
    if (shapes[j].Text.StartsWith("getSomeThing"))
    {
         shapes[j].Text = "doSomething()";                            
    }
}

形状[j]。Text = "doSomething()";这一行会导致异常，如何修复?

导致COMException。

异常消息为"The requested operation is disabled"。

ErrorCode = -2032465766