Application-defined or object-defined error

Hello all,

I have an excel 2010 macro that does exactly what I want it to do, but under certain conditions it will produce the error "Run time error 1004: Application-defined or object-defined error"

What is odd, to me at least, is that if I click "End", the code is carried out perfectly. Were it not for the error message, there would be no problem.

I have made a sample of the portion of code that has been giving me the problem. The idea is that the macro reads a number, "C", from the sheet "Model", checks to see if it lies within a certain range, and uses it in calculation, with the results displayed on the same sheet.

The problem only occurs when a number is entered that does not lie within the range. My question is, what sort of problem results in that error?

Any help would be greatly appreciated