Category.php results in inexplicable Warning message

On one of the blogs I manage, clicking on any of the category archive pages results in the following error message:

`Warning: Invalid argument supplied for foreach() in /htdocs/savageminds.org/www/wp-includes/
template-functions-category.php on line 129′

The strange thing is, the page otherwise loads correctly. I spent the last two hours looking around for some kind of information that might help solve the problem, but I couldn’t find anything. I went through some categories where I noticed some posts didn’t have the “parent category” listed, but changing that didn’t help anything for the categories in which those items were listed.

Any ideas?

You can see the blog here:

https://savageminds.org/

And a sample category page here:

https://savageminds.org/category/pedagogy/

Thanks!