I was just about to report this same issue.
My 2 cents: looks like the built-in sounding only sees the data available to the free tier, even if you are logged in a Premium account.
As of what to do when the data is legitimately not available, I'd suggest drawing a graph that says "no data", crossed out or something similar. I've had many times when I misread the graph as @Rubilin mentions. Showing the validity date and time would be nice, but probably too easy to miss.

@marekd thanks!
Now, I'm getting picker.getParams is not a function. Is this something that was missed as well?

@rittels ahh gotcha, I'll keep an eye out. Thanks!

@rittels the same error is happening on the skewt plugin. I'm having a hard time understanding what is breaking, if you can point me to your change I can try a fix and pull req (cc @johnckealy )