Does there exist some alternative to simulate() function in R?

Question

I am trying to model time series data with a modified version of nnetar model in R. For generating the prediction interval using simulation techniques, I am using the simulate function of R as follows:

simulate(fit, nsim = h) 

where, fit is my fitted model. If, I am running this code say for 10 times, I am getting the correct output for approximately 7 or 8 times otherwise the following error message is popping up:

Error in simulate.nnetar(fit, nsim = h) : I can't simulate when there are missing values near the end of the series.

The frequency of this error is increasing as I am increasing the number of simulations.

Can anyone suggest to me how to overcome this issue?

Thanks in advance.