Find with string-based parameters are not found

Other MongoDB Topics
1

In example:

type ExampleType string

type Example struct {
	Field []ExampleType
}

myStruct := Example{
   Field: []string{"Example"}
}

f := bson.M{
   f["MyCollectionField"] = bson.M{"$in": myStruct .Example}
}

This example not working, but if I change my type to string it works

type ExampleType string

type Example struct {
	Field []string
}

myStruct := Example{
   Field: []string{"Example"} 
}

f := bson.M{
   f["MyCollectionField"] = bson.M{"$in": myStruct .Example}
}

Should this behavior happen? shouldn’t it be mapped to the primitive type string?