If statement making error for some applications (NA/NaN argument)

I am attempting to produce a piece of code that is similar to functions such as rollapply from zoo/xts but applicable to my needs. I produced the code using some very simple sample data and everything has worked fine. But now that I attempt to run it on the edhec data I am receiving an error. I am unclear why but assume it is something to do with the if statement. Is anyone able to diagnose why I am receiving the error? Thank you! :) Code is as follows...

rm(list=ls()) #Clear environment
cat("\014") #CTRL + L

pacman::p_load(xts, zoo, lubridate)

is.even <- function(x) x %% 2 == 0 

roundUp <- function(x,to=2)
{
  to*(x%/%to + as.logical(x%%to))
}

functionTest <- function(data, window, slide){

  nyears_t = nyears(data)

  #IF statement for non-even numbers only
  if(is.even(nyears_t == FALSE)) {
    nyears_t <- roundUp(nyears_t)
    data_extend <- data

    start_extend <- .indexyear(data)[length(data)]+ 1900 + 1
    end_extend <- start_extend + length(data) - 1
    index(data_extend) <- update(index(data),year=start_extend:end_extend)

    data <- rbind(data, data_extend)

    warning("WARNING! The function has looped to the start of the timeseries. The final list(s) 
            will contain years that do not exist in the dataset. Please modify.")
  }

  nslides = nyears_t/slide

  #Matrix
  year_1 = (.indexyear(data)[1]+1900)

  start <- seq(from = year_1, by = slide, length.out = nslides)
  end <- start + window - 1 

  mat <- matrix(c(start, end), ncol = 2, dimnames = list(c(1:nslides), c("start", "end")))

  #For loop
  subsetlist <- vector('list')

  for(i in 1:nslides){
    subset <- data[paste0(mat[i,1], "/", mat[i,2])]
    subsetlist[[i]] <- subset
  }
  print(subsetlist)
}

Sample code that was used when I was making the function above:

a <- seq(from = as.POSIXct("2000", format = "%Y"), to = as.POSIXct("2008", format = "%Y"), by = "year")
a <- as.xts(1:length(a), order.by = a)
a

functionTest(data = a,
             window = 3,                  
             slide = 2)

Sample code I am testing on and receiving an error:

> data(edhec, package = "PerformanceAnalytics")
> edhec <- edhec[,1:3]
> edhec <- edhec["/2007"]
> head(edhec)
           Convertible Arbitrage CTA Global Distressed Securities
1997-01-31                0.0119     0.0393                0.0178
1997-02-28                0.0123     0.0298                0.0122
1997-03-31                0.0078    -0.0021               -0.0012
1997-04-30                0.0086    -0.0170                0.0030
1997-05-31                0.0156    -0.0015                0.0233
1997-06-30                0.0212     0.0085                0.0217
> functionTest(data = edhec,
+              window = 3,
+              slide = 2)
 Show Traceback

 Rerun with Debug
 Error in start_extend:end_extend : NA/NaN argument 
>