repeate open a db file then execute query will cause `file is not a database` error

Before using `go-sqlcipher`, I was using `gorm` + [sqilte3](https://github.com/mattn/go-sqlite3) and everything was working fine. 
After I switched from `sqlite3` to `go-sqlcipher`, I get an error at runtime like this:

> file is not a database

When do some query like:
```
db.Where().Find()
```

Where did this go wrong? any clue to resolve this probelem?
