I found many instances of this message in the Error Log. After trying a few things, it appears that this message is issued when the Store URL in Settings->General starts with 'https://'. When I changed the setting to 'http://' I stopped getting the messages.
This is isn't a big problem for me, because my .htaccess is set to convert all http accesses to https. (I run my commerce sites with all pages under https so I don't need to worry about which parts of the site are protected.)
However, https should be a valid prefix in the URL setting, and should not cause warning messages.
~~ Grey