Not knowing what you are working with, I will take a shot in the dark. I am suspicious about using as.data.frame on the original Object. What is the structure of the original Object? Can you show us the result of calling str() on it or say what function produces it?
The most useful thing you could post is a minimal Reproducible Example. If possible, make a small data set that can be fed into whatever function produces your Object then show that code plus the plotting step that fails.