Error when trying to unmarshal FIX message (FIX 5.0)

```
<2016-04-19 17:55:10.178353081 +0000 UTC, FIXT.1.1:CON->BON, incoming>
  (8=FIXT.1.19=14835=V34=143=Y49=BON52=20160419-17:55:10.17656=CON122=20160419-17:55:08.045262=req_A263=0264=5146=148=DORZ1722=99267=3269=0269=1269=210=213)
<2016-04-19 17:55:10.182104811 +0000 UTC, FIXT.1.1:CON->BON, event>
  (Disconnected)
panic: strconv.ParseInt: parsing "": invalid syntax
```
