Base64 of 33byte string created like this in python:
In [7]: ('x'*33).encode('base64')
Out[7]: 'eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4\n'

> db.foo.drop()
true
> b = new BinData(0,'eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4')
BinData(0,"eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4")
> db.foo.insert(

{b:b}

)
> db.foo.count(

{b:b}

)
1
> db.foo.ensureIndex(

{b:1}

,

{v:1}

)
> db.foo.count(

{b:b}

)
0
> db.foo.dropIndex(

{b:1}

)

{ "nIndexesWas" : 2, "ok" : 1 }

> db.foo.ensureIndex(

{b:1}

,

{v:0}

)
> db.foo.count(

{b:b}

)
1

The problem is in KeyV1Owned constructor. We need to check if len <= 32 before this line https://github.com/mongodb/mongo/blob/master/db/key.cpp#L267