R help - Mar 2010 - as.ltraj Error in sort.list(y)

I am trying to workout trajectories from GPS tracking data to run in the FPT
function. 
 
This is my code:
 
id<-data.frame (FPT[,6])
xy<-data.frame (FPT[,4], FPT[,3])
datet <- as.POSIXct (datetime)
x<-as.traj(id, xy, datet)
 
And I get this error:
 
Error in sort.list(y) : 'x' must be atomic for 'sort.list'
Have you called 'sort' on a list?
 
I have checked and x and y are both atomic so I don't understand the error. 
The sort command must be part of the function but I can't find out anything
about it online - can anyone help with this error please?
 
Thanks

Sam


      
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