Custom roll formulas on moves do not work

Summary

When I make a Move's "Roll" field = "Custom Roll Formula" and add any valid formula to the "Roll Formula" field:

Expected Result: clicking on the Move's roll button will roll using the "Roll Formula"

Actual Result: clicking on the Move's roll button does nothing in the UI and in Firefox's console I see this:

Examples of Roll Formulas I tried: d100, 2d6, d6, d6+@stats.head.value, d6 + @stats.head.value