DigitRecognitionRNN.cs error #25

System.ArgumentNullException: Value cannot be null. (Parameter 'first')
at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
at System.Linq.Enumerable.Concat[TSource](IEnumerable1 first, IEnumerable1 second)
at Tensorflow.Optimizer.compute_gradients(Tensor loss, List1 var_list, Nullable1 aggregation_method, GateGradientType gate_gradients, Boolean colocate_gradients_with_ops, Tensor grad_loss)
at Tensorflow.Optimizer.minimize(Tensor loss, IVariableV1 global_step, List1 var_list, GateGradientType gate_gradients, Nullable1 aggregation_method, Boolean colocate_gradients_with_ops, String name, Tensor grad_loss)
at TensorFlowNET.Examples.DigitRecognitionRNN.BuildGraph() in D:\workspace\CS\SciSharp-Stack-Examples-master\src\TensorFlowNET.Examples\ImageProcessing\DigitRecognitionRNN.cs:line 100
at TensorFlowNET.Examples.DigitRecognitionRNN.Run() in D:\workspace\CS\SciSharp-Stack-Examples-master\src\TensorFlowNET.Examples\ImageProcessing\DigitRecognitionRNN.cs:line 75
at TensorFlowNET.Examples.Program.RunExamples(String key, IExample[] examples) in D:\workspace\CS\SciSharp-Stack-Examples-master\src\TensorFlowNET.Examples\Program.cs:line 91
2020-10-28 05:29:20 Completed MNIST RNN (Graph)