Bug report: Function ppois(0:20, lambda=0.9) does not generate a non-decreasing result.

function ppois is a function calculate the CDF of Poisson distribution, it should generate a non-decreasing result, but what I got is:

any(diff(ppois(0:19,lambda=0.9))<0)

[1] TRUE


Actually,

ppois(19,lambda=0.9)<ppois(18,lambda=0.9)

[1] TRUE



Which could not be TRUE.


Code is tested in both R 3.5.1 and Microsoft R Open 3.5.1.


               _                          
platform       x86_64-w64-mingw32         
arch           x86_64                     
os             mingw32                    
system         x86_64, mingw32            
status                                    
major          3                          
minor          5.1                        
year           2018                       
month          07                         
day            02                         
svn rev        74947                      
language       R                          
version.string R version 3.5.1 (2018-07-02)
nickname       Feather Spray      







PS. if set lambda=1, such bug will not appear.