How do I get a simple value from a JSON Dictionary?

My config.json dictionary has some data in in like this:

key:Data_URL value:some data URL stuff

But when I try to load the data and display it like this....

+ System: On start of layout

-> AJAX: Request config.json (tag "load config file")

+ AJAX: On "load config file" completed

-> JSON: Parse JSON string AJAX.LastData

-> Browser: Log in console: JSON.Get("Data_URL")

..it just prints "0"

I expected it to print "some data URL stuff"

What's happening?

Thanks

Simon