Every query fails with CURSOR_NOT_FOUND

Setup: simple ReplicaSet, 1 master, 2 secondaries, all of them running on localhost, 27017 to 27019.
MongoDB version: 2.0, Ruby driver version 1.4.0
DB connections: ReplSetConnection with safe => true.

Any query I execute fails on the first record with CURSOR_NOT_FOUND, even when calling #find with a block and passing :timeout => false
Inspecting the cursor shows this:
<Mongo::Cursor:0x3ff96ff0f214 namespace='xxxx.yyyy' @selector={:a=>"b"} @cursor_id=>

The @cursor_id is not being set.