installation error

Hi, 
I am trying to install with 
library(devtools)
install_github("ISCN/SOCDRaHR2")

but there is error:
Error in ---readBuell2011 <- function(data_dir, download = TRUE, verbose = FALSE) { : 
  object 'readBuell2011' not found
Error: unable to load R code in package ‘SOCDRaH2’

How to solve this problem?

thank you!