NA for 1 particular date when converting dates from "character" format to "POSIXct" with as.POSIXct

I'm converting a string vector to date format with as.POSIXct(). Here is the strange thing:

as.POSIXct("2017-03-26 03:00:00.000",format="%Y-%m-%d %H")

#Gives

"2017-03-26 03:00:00 CEST"

#While

as.POSIXct("2017-03-26 02:00:00.000",format="%Y-%m-%d %H")

#Outputs
NA

This is really confusing and frustrating. It seem like the function really doesn't like the specific time: 02:00:00.000

Copyright Notice：Content Author:「Niltzable」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/60439346/na-for-1-particular-date-when-converting-dates-from-character-format-to-posix