I had exactly the same error. I spend the evening to find out what was wrong with my code. I thought that it was because of my version of Laravel (5.6.). And finally, I just had to kill the tinker process and restart everything, and it did the trick.
If I change my code in the model, for example, I want to display the completed tasks (put 1 instead of 0), I change the code, then I need to kill the process, and restart it to get what it is expected. Otherwise, it doesn't work.