Mirror joints and weight map symmetry--SOLVED ALREADY!

3WC

Any tricks or  tips about how to make this work?  Because I can't seem to get it to do anything.  I've tried all settings, my labels are exactly the same for left and right except for L and R--

While just typing that, a possibility occurred to me.  I have L and R as suffixes, and the mirror joints specifies prefixes.  Maybe that's the problem.

EDIT:  Yep.