Function Program to display an error message without a mesage box

I have a simple function program to calculate the inverse sine of a function. The one thing that baffles me is you can only calculate it using numbers between -1 and 1. I need to show an error message when the user tries to use like a "2" without using a message box. My code will be after this and I tried an IF conditional statement and it illustrates what I am trying to do. Naturally, what I i tried did not work.

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Assuming this is some kind of educational exercise, and that the intention is to call the function from a spreadsheet (as noted, you could use the builtin ASIN() function), I will use Excel's built in error constants with the cverr function. So a statement like

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will cause the function to return the #NUM error. If I remember right, to do this, it seems like you have to have your function defined as type variant.

My professor stated that there was but he wants us to take it and convert it to degrees. Do you think I could just take the excel function and then in my program just convert it to degrees.? I believe the error statement that MrShorty has stated it was I am looking for. Thank you

brandon