[R-sig-Geo] What does the error "ERROR in .calcTest(x[1:5], fun, na.rm, forcefun, forceapply): cannot use this function" mean in R?

[Bandrush Barda]

I have a problem with lowess function to predict NDVI as a function of time
by polynomial regression in R. NDVI image is 8-day image from Apr to Oct in
Julian date of 97 to 297. Here is the scripts I run:
 
#Created a time vector to act as x
time<-sort(sample(97:297, nlayers(ndvi2000crop1)))

#locally-weighted polynomial regression of ndvi ~ time
t.lowess <- function(x,...) { if (is.na(x[1])) {NA} else {lowess(x,y,...)$y
} } 
f.pred <- calc(ndvi2000crop1, t.lowess)

When I calculate the function on image by doing the last step of this code,
an error appeared something like this "Error in .calcTest(x[1:5], fun,
na.rm, forcefun, forceapply): cannot use this function". Can you tell me
where this error comes from and how to solve it? 

Thank you




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