What if next gererated url can be guessed in url shortener service?

I've written a url shortener, which trans a url like http://ift.tt/2mvtCZe to http://my.service/id then redirect request on shortened url to origin url. There are lots of url shortener services like goo.gl, bit.ly. The id generated by them is like random. In my implementation I use self add number and base36(0-9a-z) as id. The id generated is like 10001, 10002, ..., 10009, 1000a. The id is sequential, if you generate one id, you will know the next one. I wonder if it would cause any problem?