"Take Screenshot" response format

Hello, I am dealing with my own “/session/{session id}/screenshot” implementation on the server side and getting “unsupported format exception” on appium client side. I am returning such this:

var returnValue = “data:image/png;base64,” + base64String;

return Ok(new TakeScreenShotResponse
Value = returnValue,
SessionId = request.SessionId,
Status = ResponseStatus.Ok

Is returnValue format or response mallformed, please?

Thank you.