Acrobatic Dodge & Criticals?

In our last game, one of the PCs attempted an Acrobatic Dodge (Exploints, p. 48, and Campaigns, p. B375). He rolled an 18 on his Acrobatics roll.

What happens?

The player agued, nothing except a -2. An 18 is a failure, and a failure is a -2 to Dodge. It says that explicitly in both referenced ryles.

I allowed that, but I'm still not convinced.

The argument in favor of the player's position is that the rule doesn't specify anything about a critical success or critical failure, so therefore they don't have any special effect.

But does the lack of a specific result for a critical failure overrule this basic, underlying rule for making skill rolls? Success Rolls (Exploits, p. 5-7, Campaigns p. B343-348) calls out what constitutes a critical success or critical failure. Does the lack of a specific result for critcial success or failure mean there are no critical successes or critical failures?

For my game, I am willing but not terribly happy to have Acrobatics rolls for Acrobatic Dodge insert the concept of "only skill rolls with specified critical results have criticals" into the game. I would restrict it only to rolls in this case. Largely, though, I'm not sure I like the implications . . . but for DF Felltower, I'll go with it and say it's only for this case and the related case of Aerobatics.