AppleScript checking if folder exists causes “AppleEvent handler failed” error -10000

AppleScript on macOS Mojave and can’t work out why this script gives the expected “TRUE” on the first folder, but instead of “FALSE” on the second folder it aborts with the error message:

[error “Finder got an error: AppleEvent handler failed.” number -10000 from folder (file “Felix HD:Users:Billy:TESTING:FOLDER2”]

tell application "Finder"      set targetFolder to POSIX file "/Users/Billy/TESTING/FOLDER1" -- Existing folder      if exists folder targetFolder then         say "TRUE"     else         say "FALSE"     end if      set targetFolder to POSIX file "/Users/Billy/TESTING/FOLDER2" -- Non-existing folder      if exists folder targetFolder then         say "TRUE"     else         say "FALSE"     end if  end tell