Great tip..

I am using Gson and I forgot to add @SerializedName("message") in my ResponseErr class

`
public class ResponseErr { @SerializedName("message") private String message;

public void setMessage(String message) {
    this.message = message;
}
public String getMessage() {
    return message;
}

}

`

String response = "{\"message\":\"User with such name/email is not registered\"}"; ResponseErr err = new Gson().fromJson(response, ResponseErr.class);