forecast_meta_M4() not running as expected

[rachael-stone]

Objective: trying to run the reproduction example under "Simple Forecasting" section

Input:
reset.seed(10-06-2019)
truex = (rnorm(60)) + seq(60)/10

#we subtract the last 10 observations to use it as 'true future' values
#and keep the rest as the input series in our method
h = 10
x <- head(truex, -h)
x <- ts(x, frequency = 1)

#forecasting with our method using our pretrained model in one line of code
#just the input series and the desired forecasting horizon
forec_result <- forecast_meta_M4(model_M4, x, h=h)

Output:
Error in is.constant(y) :
(list) object cannot be coerced to type 'double'.

Any context as to why forecast_meta_M4() is not working is greatly appreciated.