This is just one of a few exceptions to that rule.
Other examples of contradictions to the rule include:
- A vast understanding
- A great happiness
- A brilliant patriotism
With such contradictions, it is surprising that such a "rule" even existed (afterall, is it not true that you have disproved a rule when exceptions occour).
Nevertheless, in many places such a rule is encouraged, for example: ESL Guide to Countable and Uncountable Nouns
Very good point, Niall. I was able to think of many more uncountable nouns that can't take an article than can. I suppose that we native speakers don't make mistakes in our usage of articles, and that's why I never thought of that rule (if it is a rule). I'm going to think about this some more.