"Exception while assigning Document ID" error in ULS logs, but Document ID still gets assigned

I'm noticing the following error in the ULS logs that says:

Exception while assigning Document ID: Unable to cast object of type 'System.Int32' to type 'System.String'.

The weird thing is that the Document ID still gets assigned properly, but this error still shows up thousands of times in the logs. It also only appears to be happening for a specific web application - and there are numerous other web applications that use the Document ID Service feature with no issues like the one above.

Any thoughts on what could be causing this?