cant write string in middle of text

Hi Guys i tried to add something to middle of a text file like this :

fs = openFile "D:\Projects\Zaeem\Exports\Map\info.txt" mode:"a+"
skipToString fs "line3"
skipToNextLine fs
print "Hello" to:fs
close fs

my text file is like this :

line1
line2
line3
line4
line5

and i want to add "hello" after line3 text. but it always add the text to the end of the file:

line1
line2
line3
line4
line5
"Hello"

anyone know what is the problem ? thanks