If you look at the tutorial I think you will see that you are using "txt" where "text" is required. "text" is a property of a textfield, identifying the text that the textfield holds. I don't know if you are doing something where the num1 object contains another object named txt, but I think my first sentence catches the problem.
Thanks very much I knew it was something simple I was missing.
The implicit coercian was also easily rectified by changing calc variable type.