fdfcbfd040
The problem here was that the "frame_start" and "frame_end" RNA properties of the Stepped FModifier were shadowing/overriding "frame_start" and "frame_end" properties of the base FModifier. As a result, when the range() callback for the In/Out parameters (defined as part of the base FModifier) checked it's start/end properties, they were always still zero, meaning that the acceptable range for the In/Out parameters was 0 -> 0 = 0. Note: If you've got old files with this problem, you'll need to manually click on the frame_start/end properties to flush out the old values. It's probably not worth the effort of applying a version patch for this (given that this modifier is not one of the most often used ones AFAIK).