Help browser: Bad arguments for message 'open' to object 'pd'

Here's one way to reproduce this:

• Open the help browser.
• Click the folder icon.
• In the Open File dialog, open any help patch.
• Click the folder icon again.
• In the Open File dialog, click Cancel.

This always gives the error message error: Bad arguments for message 'open' to object 'pd' in the console. I can reproduce this on both Linux and Mac.