# XO0425: a field type derive Xml cannot map

`derive Xml` generates Go's Marshal and Unmarshal for the struct from its field types. A field of another type (a Map, a Time, a Bytes), a struct with type parameters, or a nested struct without `derive Xml` cannot be mapped. Change the field, or give the nested struct `derive Xml`. See core 2.9.

## Example

```xo
struct Page {
  meta: Map[Str, Str]
} derive Xml
```

## Fix

```xo
struct Page {
  title: Str
} derive Xml
```

Run `xo explain XO0425` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).