Brownian Bridge Movement model error

I'm sorry for cross postings, as I'm not sure where exactly to ask 
this question:

I am trying to estimate a Brownian Bridge kernel home range using 
adehabitatHR. I created

I created a traj file as below:

bobtraj<-as.ltraj(xy=bob_locs_ss[c("x","y")], id=bob_locs_ss$id, 
date=bob_locs_ss$newdate, burst=bob_locs_ss$burst2)

Then tried to create a kernelBB as follows, with a reference grid 
around the animal's home range:

bob_bbss<- kernelbb(bobtraj, sig1=4.0811, sig2=50, grid = refgrid, 
byburst = TRUE, extent = 0.2)

I then got this error:

Error in as.vector(gr) : no method for coercing this S4 class to a vector

I didn't get the error and the function worked when I ran kernelBB() 
without byburst=TRUE. However, I would like to keep this specification 
in the model, as my data were collected at high-frequency with two 
days on and two days off, and without the burst=TRUE, the kernelBB95 
looks quite oversmoothed.

Can anyone provide any advice on how to fix this error?

Thanks for your help,

Leila Brook