Querying an document like {str: 'something', obj: {sub1: ObjectId('123'), sub2: 456}} returns nothing while pure strings or integers works well

Mon Jan 17 19:01:06 [conn2349] query: gaoren.test

{ obj.sub1: ObjectId('4d3421722973eb6246140000') }

Mon Jan 17 19:01:06 [conn2349] used cursor: 0x15de360
Mon Jan 17 19:01:06 [conn2349] query gaoren.test reslen:36 nreturned:0 0ms

Mon Jan 17 19:01:06 [conn2349] query: gaoren.test

{ obj.sub2: 456 }

Mon Jan 17 19:01:06 [conn2349] used cursor: 0x168a5c0
Mon Jan 17 19:01:06 [conn2349] query gaoren.test reslen:115 nreturned:1 0ms

Mon Jan 17 19:01:06 [conn2349] query: gaoren.test{ obj: { $elemMatch:

{ sub1: ObjectId('4d3421722973eb6246150000'), sub2: 456 }

} }
Mon Jan 17 19:01:06 [conn2349] used cursor: 0x15d1720
Mon Jan 17 19:01:06 [conn2349] query gaoren.test reslen:36 nreturned:0 0ms