Package check result: ERROR

Check: examples, Result: ERROR
  Running examples in ‘gammaFuncModel-Ex.R’ failed
  The error most likely occurred in:
  
  > base::assign(".ptime", proc.time(), pos = "CheckExEnv")
  > ### Name: pk_calculation
  > ### Title: Function that returns a data frame for Tmax, Cmax, half-life,
  > ###   AUC and AUCInf for metabolites
  > ### Aliases: pk_calculation
  > 
  > ### ** Examples
  > 
  > require(gammaFuncModel)
  > require(dplyr)
  Loading required package: dplyr
  
  Attaching package: ‘dplyr’
  
  The following objects are masked from ‘package:stats’:
  
      filter, lag
  
  The following objects are masked from ‘package:base’:
  
      intersect, setdiff, setequal, union
  
  > ## No test: 
  > df <- data.frame(
  +  ID = rep(sprintf("%02d", 1:10), each = 9 * 3),             
  +  Time = rep(rep(1:9, each = 3), 10),                      
  +  Diet = as.factor(rep(1:3, times = 9 * 10)),             
  +  Age = rep(sample(20:70, 10, replace = TRUE), each = 9 * 3), 
  +  BMI = round(rep(runif(10, 18.5, 35), each = 9 * 3), 1)     
  + )
  > metvar <- paste0("met", 1:10)
  > n_rows <- nrow(df)
  > concentration_data <- sapply(1:10, function(m) {
  +  shape <- runif(1, 2, 5)     
  +  scale <- runif(1, 1, 3)     
  +  rgamma(n_rows, shape = shape, scale = scale)
  + })
  > colnames(concentration_data) <- metvar
  > df <- cbind(df, as.data.frame(concentration_data))
  > covariates <- c("ID", "Diet", "Age", "BMI")
  > mods <- generate_models(df = df, met_vec = metvar, covariates = covariates, graph = 'None')
  > result <- pk_calculation(
  +   df = df, 
  +   met_vec = metvar, 
  +   models = mods, 
  +   grp_name = "Diet", 
  +   covariates = covariates
  +  )
  Error in UseMethod("fixef") : 
    no applicable method for 'fixef' applied to an object of class "NULL"
  Calls: pk_calculation -> precompute_f_list -> generate_f_function -> fixef
  Execution halted
  Examples with CPU (user + system) or elapsed time > 600s
                     user system elapsed
  diffGrpResponse 602.524    0.7 603.316