Why does Caret featurePlot returns returns NULL? I am trying to use featurePlot of caret in R. 

This is the code I am using: 

trellis.par.set(theme = col.whitebg(), warn = FALSE) 
caret::featurePlot(x = df[, 3:6], 
            y = df$ud, 
            plot = "scatter")


where all the columns of df[, 3:6]are of numeric type, while df$ud is a binary factor.

I also have installed the library ellipse as suggested elsewhere. 

However, I keep getting NULL. 

Any idea why?
I had this problem due to my y variable not being a factor which is required. call:

typeof(df$ud)


make sure it is not interpreting the binary as character or numeric but indeed ad a factor.