Golang Error Types are empty when encoded to JSON

I'm trying to encode some JSON for a REST api, everything is working fine except for some errors. For example, with this struct:

type TemplateResponse struct {
    Message string
    Error   error
    Template Template
}

Encoded with this data:

res := TemplateResponse{"Template not found.", fmt.Errorf("There is no template on this host with the name " + vars["name"]), Template{}}
json.NewEncoder(w).Encode(res)

Returns:

{
  "Message": "Template not found.",
  "Error": {},
  "Template": {
    "Name": "",
    "Disabled": false,
    "Path": "",
    "Version": ""
  }
}

I'm getting this seemingly randomly across my application, where 'error' types are being returned as empty. Any ideas?

Thanks!