I did try reproducing the issue, and wasn't able to. You can try one of these two things.
1) As KSSV suggested, you can check what the which plot command returns. If it is a custom function which is in the same folder as pwd, then MATLAB will call that function first and hence you might be getting the above mentioned output.
2) It is also possible that you had made a mistake in the syntax on your command window and what you typed here is correct. Whilst, it is unlikely, I would suggest you try it again.
Hope this helps,