commit-msg hook and merges

Hi. 

I'm running a speciallized commit-msg hook to help me fill out commit messages. This all works nicely for alle commits except for merges.

What I normally do to circumvent this is this :

$ git merge somebranch

<here I append to autogenerated message with my own text>

$ git commit --ammend

<I just save the message without changes and the commit-message is run throught my filter>

Is that intentional or an error ?