System Information
Operating system:
Graphics card:

Blender Version
Broken: (example: 2.80, edbf15d3c044, master, 2018-11-28, as found on the splash screen)
Worked: (optional)

Short description of error
Custom node attributes don't interpolate between keyframes. In the attached gif, the weight parameter is set to float.

Exact steps for others to reproduce the error

Based on the default startup or an attached .blend file (as simple as possible).