no applicable method for 'st_as_sfc' applied to an object of class "NULL"

[Zorro Katz]

Using opentripplanner 1,  I've set up my map.   I have a simple query

route2 <- otp_plan(otpcon, + fromPlace = c( -75.1 , 39.9 ), + toPlace = c( -75.2 , 39.8), + mode = c("WALK"))

When I run it on my pc, it works - usually. But sometimes after processing the results it says 'Error in UseMEthod("st_as_sfc"):

no applicable method for 'st_as_sfc' applied to an object of class "NULL" . On my other machine, this happens every time.

How can I identify the problem , and is there any solution?