Gson crashes app when receiving empty String instead of object

Question

I trying to populate a response object that looks like this using Gson:

public class RegisterUserResponse extends BaseResponse {

  @SerializedName("tokens")
  private Tokens tokens;

  public RegisterUserResponse() { }

  public Tokens getTokens() {
      return tokens;
  }
}

My Gson builder looks like this:

RegisterUserResponse response = getGson().fromJson(jsonResponse, RegisterUserResponse.class);

When I receive a JSON that contains the object everything's fine. The problem begins when "tokens" return empty, like in this JSON:

{"tokens":""}

and I get this exception:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line # column #

Which I do understand, but don't know how to avoid.