careful with spaces, do you ever write a sentence putting a space before everything else? or do you usually put a space as first character?
spaces usually go only after words and punctuation
I think in this challenge, myStr should have a value of This is the first sentence. This is the second sentence.
So as @ilenia pointed out, I suggest removing the first space in the first sentence and also replacing the word "string" in your second sentence with "sentence".