vimscript function give "E488 trailing characters: end if" error (NERDTree)

I am trying to function to toggle the size of my nerdtree split view. However I am getting "E488 trailing characters: end if".

function! ToggleNTWidth()
    if exists("b:NERDTree")
        if g:NERDTreeWinSize == 31
            let g:NERDTreeWinSize = 50
        else
            let g:NERDTreeWinSize = 31
        end if
    end if
endfunction

I have tried putting in exe in front of statements involving computation and converting segments into string `= 50'. This did not help.

In Vimscript, the closing keyword is a single word (:endif, also :endfunction, :endwhile, etc.)

Learn how to look up commands and navigate the built-in :help; it is comprehensive and offers many tips. You won't learn Vim as fast as other editors, but if you commit to continuous learning, it'll prove a very powerful and efficient editor. Vim would have answered this one simply by typing :help :end and then doing tab completion by pressing Tab, or pressing <C-D> to show all candidates (:help c_CTRL-D).