This is a very minor bug I ran into. In certain text fields that set an object's Opacity value, the character limit is 5 characters, which is not enough to do certain mathematical operations, like 12.5/2. If the value is a decimal number with two places, no math can be performed at all.

The 5 character limit applies to the control that appears in the Control panel when selecting an object, and the control that appears in the Effects panel when selecting an object.

The only field that allows enough characters to perform that operation is the one found in the Effects window, which can be accessed by selecting an object, clicking on the Fx icon, and selecting the first option, Transparency.