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This is only a problem with the relational database backends -  
they may not sort or group on a multilink property. There is no 
really easy fix for this - except perhaps for someone to spend 
some time looking into handling sort/group for Multilinks in the 
rdbms backends. 
 
If it's really annoying you, feel free to hard-code the list of 
sort/group properties that are valid for your tracker.