Querying for in string array in mgo

I have an element that looks like this:

{"Name":"test name","DBType":0,"UserName":"test user","Password":"","Host":"test host","Port":"123","DBName":"test schema","Options":"test options","Groups":["test1"]}

I want to be able to query the cataloug of Datasources for those which are assigned to a particular group.

My code for this is:

var d []Source
q := bson.M{"Groups": bson.M{"$in": [1]string{groupName}}}
findErr := c.Find(q).All(&d)

However - I get no error and no results.

I've also tried

q := bson.M{"Groups": groupName}

with the same result.

//Source describes a data source
type Source struct {
    Name string
    DBType      uint
    UserName    string
    Password    string
    Host        string
    Port        string
    DBName      string
    Options     string
    Groups      []string
}

I'm stuck - I am puzzled! Any help appreciated.

Update - I tried

{"Name":"test name"}

and this returns 0 items. Yet if I set to nil I get stuff.