After collocation, images will not be saved in geotiff

Dear all,
I used collocate for collocation two images. After collocation, we have flages. When I want to save the result in geotiff (I used below steps) but I do not know why these steps work for Mater image but do not work for SLAVE image?



Figure 3.

When I delet ‘Valid pixel expression’, then I can save SLAVE image too but why it is like that?