实际显示时，我的控件大小如何为NaN？

I have a Grid which contains an Image in one of its columns. The image itself does not have any Width or Height set, but its size is correctly controlled through the ColumnDefinition set. From this I would assume that the image controller actually has a Width and Height set, but when I try to bind another element to its Width and Height it doesn't work. When debugging it turns out that the value of Image.Height and Image.Width are NaN. Why? I have a Grid which contains an Image in one of