grepFromString with caseSensitive=False never executes the body of the for beyond the "if caseSensitive" statement (line 12)

test.py

def grepFromFile(filename, string):
	content = run('cat' + filename, capture=True)
	return grep(content, string)

def grepFromString(haystack, needle, caseSensitive):
	result = None
	if(caseSensitive):
		needle = needle.lower()
	s = StringIO.StringIO(haystack)
	for line in s:
		if caseSensitive:
			line = line.lower()
        if needle in line:
            result = result + [line, ]
	return result

def main():
	res = grepFromString("""
		hello
		Kill
		Me
		Please
		""", "me", False)
	print res

if __name__ == "__main__":
    main()