Shutter mode Sony a6000 – Stack Exchange Photography

When shooting in Shutter mode, the aperture indicator on the screen can not stop flashing when I set an ISO value.

I use a fixed lens Sigma 60mm F2.8.

I've tried taking a picture in smart mode, then replicate the values ​​in shutter mode, but the aperture indicator keeps flashing even though I'm increasing / decreasing the shutter speed.

The problem does not occur if I put ISO on Auto but I want to avoid it when shooting in low light ….