Many cameras will automatically increase the ISO if there's too little light.
You may be surprised when you discover your photographs of an illuminated fountain at night were taken at ISO 1600, when you had the ISO set to 400.
If you turn this feature off, and there's too little light, Lo will blink in the viewfinder.
You can now decide, not the camera, to increase the ISO.
If you don't want the added noise from using a high ISO, you can change the chosen aperture or shutter speed.
Or, if your aperture is already wide open, letting in the most light, such as f/4, you can switch to a faster lens.
A faster lens has a wider lens opening, such as a 50mm f/1.4 or f/1.4.