Password required, but none set

I had a strange thing happen today when I upgraded a 2960G from IOS 12.2 to 15.0. After booting the upgraded IOS, I logged in, entered the enable command, and was surprised to get this error:

2960>en

Password required, but none set
Password:
2960#

Everything seemed to work fine, but it was a little odd. I did discover that the following command resolves the error:

aaa authentication enable default enable

Which is also odd, because the documentation states:

If the default list is not set, only the enable password is checked. This has the same effect as the following command:

aaa authentication enable default enable

I’ll go with the explicit statement because the error message, while it may be spurious, makes me uncomfortable.

FIN