There was an email thread [1] in which Jonas proposed this change and we had agreed to the following:

>> I propose that we remove the requirement that we have today that if 
>> an indexed property exists, it has to contain a valid value. Instead, 
>> if a property doesn't contain a valid key value, we simply don't add an entry to the index.
>> This would of course apply both when inserting data into a 
>> objectStore which already has indexes, as well as when creating 
>> indexes for an object store which already contains data.

We need to add this to the spec.

Israel

[1] http://lists.w3.org/Archives/Public/public-webapps/2011OctDec/0534.html