Facebook error: "Errors while loading page from application Runtime errors: URLExceptionEmpty URLs not allowed here"

I got this error while coding a canvas page:

"Errors while loading page from application Runtime errors: URLExceptionEmpty URLs not allowed here"

It turns out that I had an image tag that I wanted to start empty and then set the src at a later time using javascript:

<img id="myimg" src="" />

But the error above gets thrown when you try to leave the img src blank. So as a workaround I've placed a transparent spacer image instead and now it works fine...