Shell:
SECONDARY> rs.slaveOk()
SECONDARY> db.foo.getIndexes()
[
{
"v" : 1,
"key" :

{ "_id" : 1 }

,
"ns" : "test.foo",
"name" : "id"
},
{
"v" : 0,
"key" :

{ "x" : 1, "y" : 1 }

,
"ns" : "test.foo",
"name" : "x_1_y_1"
}
]
SECONDARY> db.foo.reIndex()
{
"nIndexesWas" : 2,
"msg" : "indexes dropped for collection",
"errmsg" : "exception: no index name specified",
"code" : 12523,
"ok" : 0
}
SECONDARY> db.foo.getIndexes()
[ ]

Secondary server log:
Wed Sep 14 10:05:59 [conn5] CMD: reIndex test.foo
Wed Sep 14 10:05:59 [conn5] assertion 13435 not master and slaveok=false ns:test.system.indexes query:

{ ns: "test.foo" }