What does Rom. 4:15b mean in context: "And where there is no law there is no transgression."

In Romans 4:1-15, Paul has argued that Abraham was justified by faith apart from works of the law and apart from circumcision. Then comes 4:15a, "because the law brings about wrath;" But then he says in 15b, "And where there is no law there is no transgression." Why was this 15b written? What is the point after what was said before? I don't see how "transgression" enters the flow of text.