Here is an example of what happens when one makes a mistake.
There is a good implementation of a function, and there is one where a sign went wrong. This is a rather common occurrence.
Check carefully if you can spot the difference. You can hardly tell the two plots apart 
You can only tell which one is right if you have some expectations of what the end product is supposed to look like.
Good coding
Bad coding
To me it is quite clear that the left picture is right (zero) and the right one is wrong (infinity, does not fit the frame).