mongod OOM in map_reduce

#!/usr/bin/python

from pymongo import Connection
from pymongo.code import Code

conn = Connection().lab
col = conn.usage

def query():
    conn.report.drop()
    m = Code(""" function() { emit({ user : this.user }, { count : 1 }); } """)
    r = Code(""" function(key, vals) { return {count: 1}; } """)
    q = { 'action' : 'push' }
    col.ensure_index("action")
    col.map_reduce(m, r, "report", query = q)

query()