If you access a container directly (but I'm only assuming you do) without first checking its length, you will end up with, in the best case, odd results or, as in this case, failure. Typically, using an iterator is the safer bet since you then get well defined situations if it's, for example, an empty container. So, your call to the QNetworkAcessManager instance didn't behave as you expected it to. Check what resources this class relies on. How do you instantiate it? What call did you do? In regards to documentation, I'm not more in the know than you. Qt is a framework with lots of interesting things in it and quite a few hidden dependencies that you need to understand. Have a thorough look at the Qt documentation for network connections and look at the examples (one more time) and try to find if there are differences from your code.