yth_filter() fails with p =1

[SvN-HEC]

To reproduce the problem:

data(GDPC1) 
# the following example works fine
gdp_filter <- yth_filter(100*log(GDPC1), h = 8, p = 4) 
# change p = 4  to  p = 1
gdp_filter <- yth_filter(100*log(GDPC1), h = 8, p = 1) 

produces the error message
Error in eval(predvars, data, env) : object 'xt_1' not found

Now $p=1$ may seem counterintuitive ... but it makes sense in the context of what I'm trying to do. (Trust me....)