Text Field Custom Calculation Script Won't Save/Stay Selected

I have a Text field named "MyField". I'd like to add a Custom Calculation Script to it. Every time I select the edit, enter the code, and then click OK, it does not save the code and then reselects the default "Value is not calculated" option. It will even happen in fields that I have already saved previously and that did once work. When returning to them sometimes, I will see the selection change from "Customer Calculation Script" to "Value is not calculated" as soon as I activate that Calculate tab. Does anyone have any idea what causes this and/or how to avoid/correct it. I have tried searching all over Google and can't seem to find anyone else that has brought up this issue.