densityplot(~x+y) for vectors of different lengths does what?

densityplot(~x+y) does what I expect it to do if x and y have equal length.  I know how to get what I want if x and y have different lengths.   But what is this actually doing if x and y have different lengths?    The relevant example is
 
x=rnorm(10)
y=rnorm(50,1)
densityplot(~x+y)