GSON is trying to deserialize a String to double

I'm using Retrofit with a GSON converter to obtain JSON from a RESTful Web Service and when GSON tries to convert a JSON object to POJO I'm getting the error:

retrofit.Converter:conversionException: com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: Invalid double: "610/182"

This element corresponds to a String variable in my Java Object:

@Expose
@SerializedName("code")
private String code;
...

And the JSON Object is:

{"code":"610/182", ...}

I added a converter to my RestAdapter like this:

    restAdapter = new RestAdapter.Builder()
            .setEndpoint(URL_BASE)
            .setClient(new OkClient(getClient()))
            .setConverter(new GsonConverter(gson))
            .setLogLevel(RestAdapter.LogLevel.FULL)
            .build();

    api = restAdapter.create(ApiRest.class);

Why GSON is trying to convert a String into a double?