Error when adding a block from a custom module to a region

I've poured over my code, but can't for the life of me figure out what could be causing this. As soon as I post this, I will invariably find the issue, but ... for now ... :)

I have a custom module that creates a block. And it works fine, but when I add the block to a region and save blocks, I get this in my error log:

Recoverable fatal error: Argument 1 passed to Drupal\\Core\\Block\\BlockBase::setConfiguration() must be of the type array, null given, called in /home/www/testsite/core/lib/Drupal/Core/Plugin/DefaultSingleLazyPluginCollection.php on line 88 and defined in /home/www/incurs.us/core/lib/Drupal/Core/Block/BlockBase.php on line 68, referer: ....

Now, even though I get the error, I can go to my admin config form, and it works fine. The block itself also works fine. Any ideas what type of thing would be causing that?

Thanks in advance for any guidance!