error msg for Visitor/non-member

Hi! When looking at my site as a "Visitor", I'm encountering this error: "Warning: Missing argument 2 for membershippublic::show_moretag_protection() in /home/content/d/e/"(etc.)

Is there a setting I need to change? I currently have Visitor access level set to Negative Rule - More Tag. (I want visitors to see a teaser for each protected post, but not be able to see the protected content). I don't get an error message if I just set up Visitor to not be able to see posts of a certain category.

I've used "Verify Membership Tables" and it says everything is fine.

Thanks for your help!