Map Reduce queries routed to secondaries, subsequently fail

We have a map reduce query that was previously working fine in 1.6.x that under 1.7.0 appears to be routed to a secondary and then subsequently fails. The map reduce uses an output merge strategy to an existing collection. The failure we get is:

Database command 'mapreduce' failed: (errmsg: 'not master'; ok: '0.0').