Error adding entity to schedule

[benlaurie]

Checklist

• I checked for similar existing issues (both open and closed) before posting.
• I will participate in further discussion about this issue and can help by testing (if requested).

Card Version

v3.2.13

Component Version

v3.3.8

Bug description

I attempted to add an extra entity to an existing schedule. First of all, I get "Some settings will be lost by this change. Do you want to continue?", if I say yes and then try to save, I get:

Something went wrong!
Message format incorrect: length of value must be at least 1 for dictionary value @ data['timeslots']

Response error: 400

Please report the bug.

Steps to reproduce

I don't know, though it reliably goes wrong in my case.

Additional info

No response