Unable to match data on map[string]struct

I'm having an issue when executing a JMESPATH query on a `map[string]customType`, here's the code :
```go
type testStruct struct {
Value interface{} `json:"value"`
}

func main() {
test1 := map[string]interface{}{
"entry": testStruct{Value: 1},
}
result1, err := jmespath.Search("entry.value", test1)
fmt.Printf("\nerr = %v : res = %+v\n", err, result1) // displays err = nil : res = 1

test2 := map[string]testStruct{
"entry": testStruct{Value: 1},
}
result2, err := jmespath.Search("entry.value", test2)
fmt.Printf("\nerr = %v : res = %+v\n", err, result2) // displays err = nil : res = nil
}
```

As you can see, a query on a `map[string]interface{}` behaves as expected, whereas `nil` is returned when executing the same query on a `map[string]testStruct` (adding a pointer to `testStruct` results in the same output).

Has anyone ever experienced such a problem ?